I just painted a case for the first time a few months ago, and definitely learned some lessons. Here are some simple instructions on the procedure:
1) Start off by sanding/scuffing the entire surface you'll be painting. Start with a rough grit, then switch to a fine grit. When done sanding, I would wipe it all off with a damp paper towel. Finally, wipe once more with a dust-free clothe to ensure no paper-towel fibers are left on the surface.
2) Buy a primer in the same brand as the paint. Trust me, this makes a big difference. Apply 1 or 2 coats
3) The paint should be applied in very light coats (I did anywhere from 3 to 5 coats), and you should wait 5 - 10 minutes between coats. Look for some youtube videos on proper technique here as it is very important. You should be applying in a sweeping motion from side to side, starting and stopping your spray beyond the edge of the surface being painted.
4) The most important step, and the step that I know I didn't always follow, is that once you've finished painting, do not touch the pieces for a MINIMUM of one hour. Even after that hour, the piece should be handled only with care and only if it is neccessary to move it to a different location. You will be tempted to pick it up and check out your handy work; you will be tempted to slap it on to the case and see how awesome it looks - but that would be the biggest mistake. The paint needs to fully cure before you should start doing anything with the piece, and the process of curing takes more than that initial hour. Trust me: even if it looks and feels fully dry, IT IS NOT. My best advice is to let it all sit overnight before doing anything with it.
I really can't emphasize the overnight wait enough. I got overly eager to check everything out, and although the case still looks great, there are minor imperfections as a result of my impatience that will always bother me.
Best of luck with your mod and feel free to ask any additional questions