I have done it as well. Sold quite a few to people here on this board. Testors makes a decal paper that you can run through your home printer. Then you need to spray it with Testor's Decal Bonder or else the ink lets go when you get the decal wet. They make a transparent film and a white one. I guess the white one would be good if the decal you wanted had white in it as most printers don't do white but then I guess you would have to trim it pretty close to get rid of any white on the outside that you didn't want. Since I have done mostly tattoos and irises (the cloured part of the eye-I can't paint them for crap) I have used the transparent sheets. I pick an image off the net or make one in MSPaint and then I open it in word to resize it and print a few copies on regular paper to get the sizing right. Then I print it on the decal paper when everything is just so. If you have one of those brain-dead "smart" HP all in one printers it might reject your paper (at least mine did repeatedly). I finally figured out to go to properties and pick presentation printing and select other brochure papers. That finally worked after hours of paper mismatches and repeated emails to HP Customer Support about my printer spitting my paper out. Then you just cut the decal to the required size, get it wet til it starts to slide off the paper, place it where it needs to go and seal it. I would reccommend some sort of matte sealer as I have used Future floor polish (usually all I have on hand for a water based sealer) and that stays a little shiny, even after spraying with Dull-cote. Hope that helps. If you have any more questions feel free to ask away. I have done quite a few of these and through trial and error worked a lot of the bugs out.
I could also just do it for you if you were interested. All I would need is the images you were looking to have decals made out of. Would probably do it for free just to have something I made on one of the great Seb's figures.
Mike