For this tutorial, I use two programs - Ulead Photoimpact 8, and Corel Painter 8. It's possible to make pretty wings just with UPI or any other similar vector program, and it's actually advised if you're stuck with mouse or don't want your hand to fall off from drawing an excessive amoung of little lines. ^^
1. It all starts with some sketchy lines to mark the basic shape of the wings. That way, you keep an eye on the anatomy and all that, because otherwise it can get very confusing later on.
2. Then, you start drawing in vector feathers. For a fill, it's best to use a gradient that has colors different from your background. You use gradient so that you can easily tell apart between individual feathers, so that you know what you're drawing. It's not a bad idea to use a reference to check how feathers overlap each other. When drawing them, you start with the longest feathers, because they will be underneath all other ones. Then draw the middle row, then draw the 'hand' part, then the upper rows of smaller feathers. Okay, now you have a pretty basic wing, with nice vector feathers. They look very boring though, and not all that realistic (depending also on how lucky you were with your gradient).
3. Now you tinker around with the gradients till you're happy with the general color, because it'll be the basis of everything else. Now, group the feathers - the number of groups depends on how many places the wing will be bent in. In out example, the bunny's left wing (our right) has 7 groups - 3 of them are the 'hand' part - long feathers, middle feathers, and smaller feathers, and another 4 are the rest - long feathers, middle feathers, group1 of small feathers, group2 of small feathers. The reason for this is being able to tinker with the lighting in a more effective manner, and prevents clutter of objects if you're bringing the file into Painter. Now, if there's something about those gradients you want to modify in groups, it's the right time to do so. Done? Good. Next step!
4. Select each group, rightclick on it, and pick 'merge as single object'. That merges the feathers in a group into one object. That means, you'll no longer be able to edit them as paths or each separately! Now, save the file as Photoshop file (.psd), and close it. Say buhbye to Photoimpact, we won't be needing it anymore for this.
5. Fire up Painter and open the file there. Each of your objects will now be a layer of its own, with a name 'obj-(some number)'. If you want, rename them into something more manageable. Now, select some group - might want to start with the long feathers first - and duplicate the layer. Place the new one exactly over the old one. Now, click on 'create mask from transparency', and then 'negative'. This lets you see exactly where the new layer is. Now, get a big happy eraser and delete it! The reason for doing so is getting a mask in the shape of the underlying group on a new layer. (If there's an easier way to do this, let me know!) The negative was used to make sure you have deleted ALL of the color in the duplicate layer. Why do you want to do this? See now we're going to do all sorts of detail, and if you screw something up, it's easier to just delete the faulty part or check with what there used to be, than start it all over again.
6. Okay, now get a blender. I suggest using 'grainy water 30', because it blends the underlying colors as well - most other blenders will only blend the colors on the current layer, even with the 'pick up underlying color' checked. Now, keeping in your mind where your lightsource is, draw two parallel lines on each feather, parallely to its length. It should mark the upper and lower part of the feather - and they needn't be 50/50 either! The line between those two blending lines, will be where the middle line of the feather will go. Now do this process for ALL the feathers. (Not kidding.)
7. Get some pretty light or dark color - best of all, both, so you can make a little shadow as well - and draw in all the middle lines. My favorite brush for all those things is 'wet detail brush 5' from the acrylics, because it both blends and draws very small detail quite well. If you're using Photoshop or something else at this stage, DON'T EVEN THINK of only using dodge/burn lighten/darken tools! What you'll get will be rather monocrhomatic feathers, that won't look as interesting as they'd get if you use actual color. Check reference for where they are located for different types of feathers.
--Warning! The following part should only be done if you have no attachment to your wrist, because it may become very detached to the process.--
8. Now get a few nice colors laid out, and draw a big bunch of tiny little lines going at an angle against the middle line on each feather. Make sure the lines on both sides of the middle line are perpendicular among themselves. Use lighter lines where the light hits your feathers, and darker lines where there's shadow. Mix colors, too, so that your feathers look shiny. Do this till you have detailed out all the feathers in the layer. Save.
9. And now - rinse and repeat for all the remaining layers! Keep in mind that the small feathers will definitely have their middle lines positioned differently than the large feathers at the end of the wing. The small ones usually require less lined detail as well, 'cause there simply isn't that much space on them, and their size alone creates a more detailed impression.
Well that's about all. If you have any questions, please post them in the comment area. Good luck and wind to thy wings!