Here I will explain how I draw and colour images on the computer. It's not a lesson in anatomy, just the actual drawing/colouring. Who knows, maybe you'll find it helpful!

If you'd like to see my drawing style "In action", here's a sped up version showing the process.

Tools used
I use the following tools. You can probably get the same/similar results with other equipment/programs but that's something you'll have to experiment with yourself.
Note, I expect you have used the program before. I'm not going to explain everything in the program from start to finish ;)

Setting up the image

Step 1 - New file
Create a new file. Make it BIG. At least 1000 x 1000 pixels or so. Later we will resize the image to its final web size, but always start big. For one thing, you can always make a pic smaller with kept quality, but if you enlarge it, it will look crappy as long as it's not a vector. Which this won't be. Plus when we resize it, many minor oddities like uneven lines and stray pixels will magically disappear.

Now, make a new Layer. Can't see the Layers window? Make sure "Layers" is checked under "View" in the top menu.
Then pick a paint brush (size 3-5 is normally good depending on how large your image is) and make a sketch of what you want to draw. Be sure to use all your space wisely, don't draw just in the bottom left corner or something, that defeats the purpose of having a large image...


Two layers of sketching, one rough and one a bit more detailed. I'm now ready to fill in the outline. 
I never follow the sketch exactly when I ink, but it provides me with useful help lines.

Step 2 - Sketch 
Different people use different methods for sketching. I personally tend to make a rough sketch, then decrease that layer's opacity (done in the Layers window, top right) to say 40 % or so, make a new layer and draw a more detailed sketch on top of that, then delete the the first sketch layer. This leaves you with a relatively clean sketch when it's time to ink it. How many layers of sketches you want to do depends on how secure you feel, remember the sketch should aid you when you fill it in later so be nice to yourself and give yourself all the help lines you need!
I usually make sketches in green or blue, but that's just personal preference, it doesn't really matter. It can be helpful to make a non-black outline though because it makes it a bit easier to tell sketch and outline apart in the next step.
Tip! Now and then, flip your image. Go to Edit > Transform > Flip horizontally. Why? Because the new mirrored perspective is a foolproof way to see what looks off in your image. It's a little weird just how much more clearly you'll see things like uneven proportions and such in a mirrored image! Then when you've cleaned up whatever needed to be fixed, just flip the image back.


Finished outline

Step 3 - Inking
Inking is perhaps not the best term when you're doing it digitally... ah well...
Make a new layer, pick a good size brush (I'll use size 4 for this tutorial for main outline and 3 for details, image is 1300 x 1300 pixels). I always ink in black, we can change colour later. Lower the opacity of the sketch layer so you can still see it, but also clearly see your new outline on top of it as you fill it in.
Now comes the painstaking work of drawing your outline. You will need patience for this step, and a steady hand! This is where a tablet comes in very handy... It allows you to vary the thickness of the line depending on how hard you push down with the pen, which you can't get with a mouse. But you can ink with a mouse too, it just takes longer as you may need to clean up more and switch brush size now and then.
I personally like to first make the "big" outline, then go back and fill in the details with a smaller brush, but that's up to you.

When you're doine filling in the outline, go over it again. Make sure there are no gaps and you can also fix up lines so they look nicer. You can easily do this with the Eraser tool. Don't forget to flip the image sometimes to make sure nothing looks odd!

Tip! If you know you're going to want to have a non-black outline in the finished image, consider drawing different parts of the outline in different layers. This will save you work further down the road. For instance, here I used one layer for the left cat and one for the right, and a third for both cats' facial details, because I knew each of those bits would be using different outline colours.
Tip! Be sure the outline is "closed", that there are no gaps. For instance, in the sketch above there's a gap between the ear and back in the left cat. Close all gaps like that. This will save you work in the next step.


First layer if colour has been blocked in. Save this layer, we'll also use it for shading.

Step 4 - Blocking in colour
When you're happy with your outline, delete the sketch layer, and create a new layer below the outline. Now we're going to block in colour.

Select the Wand tool - Anti-alias, Continous and Use All Layers should be checked.  Tolerance should be set as high as possible that still lets you select only the background (that is, when you click outside the outline, it surrounds the outline but doesn't go inside it). If your outline is as thick as mine in this image, you should be able to set it at 200 or so.
After selecting the wand, click anywhere in the background. It should surround the outline, but not go inside it. If it does, lower the tolerance and/or make sure there aren't any gaps in the outline. When you're happy with your Selection, go to Select in the main menu and click on Invert. Now it should have selected only the inside of your image, the cats in my case. Block in with any colour, it doesn't matter, we'll change it later.

Now, if you're smart, you will use different layers for different portions of the image, because then you don't need to worry about painting over the wrong area in the next step.
For instance, here I want the left and right cats to be in different layers. There are many ways to do this, what I usually do is this (it sounds very complicated when described, but it's really not when you do it!):
1) Duplicate the layer (in the Layers window, right click on the Layer name, Layer 1 or whatever it may say, and choose Duplicate layer).
2) Use the Lasso tool and draw a selection, in this case I want the right cat on a new layer so I will draw a selection around it. Lasso should be set as anti-aliased and with 0 feather.
3) Draw in the shape you want to separate. Now, since there's no colour outside the cat, you do NOT need to follow the whole outline closely there, just the bits where the two cats overlap.
4) When you're happy with your selection (here, I selected the right cat) go to Select and click on Invert again. Then press Delete. This will remove everything except the colour of the right cat. You now have a separate layer for it!
Tip! When using the lasso tool, learn the shortcuts for Add or Remove selection. This will make your life a lot easier. If you have selected a portion of the image, and want to add more to the same selection (instead of needing to select all over again) then simply hold in Shift. You should notice a little plus appearing by the lasso-shaped pointer. This means it will add to the already existing selection instead of making a new one. Then you can just draw in the bits you wanted to add.
If you want to remove from the selection, like if you accidentally added too much in one spot, hold in Alt instead. A minus will appear. It will then remove whatever you select next from the existing selection.

Duplicate layer > Select appropriate section > Invert selection / Delete > Right cat has his own layer of colour

Alternatively, you can of course just make a new layer and do a selection or just use the paint brush and draw in the colour that way, which is the quickest way for small details, but it's good to know how to do the duplication/selection thing, because we're going to use it further down anyway... The reason I prefer to duplicate and select/delete is pretty simple: If I had just made a new layer and drawn on that, I would need to mind the outline of the whole cat, or colour would go outside it the outline. This takes a lot of time. If you look at the image above, you can see I only needed to follow the outline where the cats met, not at all outside of that. This is because I used the original blocked in layer as my "base" instead of an empty layer that I filled in, so there's no colour there to worry about, I only isolated part of what had already been blocked in previously - saves a lot of time and work!


All colour blocked in

Step 5 - Colouring in details
When you have your basic colour layers done, click the little box right of Lock: in the Layers window. This locks the layer so transparent areas can't be painted over, just already coloured ones. No need to worry drawing outside the lines then! You can of course uncheck if it you notice you have to. Now colour in the whole image like you want it. If you want to make details of different colours/patterns, I recommend to use the Lasso tool to select the areas, instead of drawing them with the paintbrush, because it's a lot easier to get nice looking details then:

Left is using the paint brush, right the Lasso. It's a lot easier to get nice looking details with the Lasso.
Select the area with the lasso, then colour it with the paint brush set at a very large size. You can experiment with adding varying amounts of feather (Lasso setting) to get softer looks.

There are many ways to do eyes. I usually do big, bright cartoon eyes and colour them in like this:
1) Fill the whole eye with the basic colour
2) Add a darker portion at the top. This is she shadow that falls there because of the skull/eyebrows.
3) Add a lighter portion further down. This gives a bright spark. I also like to add a few more extra light sparks here and there, kind of like a pattern of the eye.
4) For the cartoon look, add some eye white. You can give it a bit of light blue at the top for shading.
5) If you want, add one or two white spots, I usually do this on a layer placed on top of all other layers, even the inking layer.
Tip! At the end, when resizing and doing the final touches, try adding a Sharpen to the layer of the eyes, for extra brightness and contrast.

How I colour eyes


One layer of dark blue shading

Step 6 - Shading
There are as many ways to shade as there are artists... I won't teach you how shading works in this tutorial, just how I add it in Photoshop. It's actually very simple.
1) First, duplicate the very bottom colour layer. You know, the teal one that included both cats? Duplicate your basic colour layer. Make sure you are on this layer when you do the next step, even though you can't see anything different in the image right now. Pick the Lasso and simply select all areas where you want shading. Then once again Invert the selection and click delete. You will now have a layer that only has colour on the shading areas.
2) Move the duplicated layer so it's above all other colour layers. Lock it, colour it solid black and change the opacity, 25 - 30 % is usually good. Now you should have shading!
Tip! You can repeat this stage and add more and more shading layers (with shading in various areas), giving darker and darker areas and thus more depth in your image, if you want. I usually like to do 2-3 layers of shading.
Tip! Solid black shading is very basic and a bit dull, especially on white areas. Experiment with using different areas. Cream or blue shading often looks very good on white areas.

You can also do the same thing but with white instead of black - and get highlights! For highlights it's often good to experiment with layer type in the Layers window (default is Normal, try for instance Overlay) for different effect. Since so much of my image is white anyway, I'm not going to bother to do it in this particular drawing.


Step 7 - Last touches
When you are happy with your shading, go to the layer(s) with the outline, lock those and, if you want, colour those too for a softer and smoother look.

Go over the image and see if you want to add any more details. Save your image if you haven't. Add a signature, and whatever else you may want to do.

When you want to save your image for the web, click on Image in the main menu and Resize. Type in the size you want (either exact pixels or percent of current size - I usually resize to ca 50 %).

If you want, try fiddling with the layers, like flattening the image and using Unsharp mask... And then...

You're done! :D

If you'd like to see my drawing style "In action", here's a sped up version showing the process.