Decals have been around for a really long time (first used around 1750). They were historically made from copper plate etchings then transferred to tissue paper or glue which was then applied to pots in a long labor intensive process. Fortunately things are much easier now. There are still quite a few kinds of decals (ceramic, glass, midrange, in glaze, enamel, metallic, etc.) most of which are waterslide and easy to apply and to have made. If you want more information on types of decals try this website...http://www.ceramicdecals.org.
We're going to be using iron decals which are technically 'in glaze' type decals. They fire at a much higher temperature than most ceramic decals so they melt into the glaze rather than sitting on the surface. Iron decals can be printed on most HP laser jet printers. They generally fire between cone 010 and 7, and the color will change depending on the temperature. Iron decals will generally be brown, sepia, or purplish, but you should test them to see how hot the decals from your printer need to be to get the color you want.
You can use any photo or document program to design your decals. cram the images together on a sheet to save paper and money. You'll need to buy special decal paper to use in your printer which can be purchased at www.belldecal.com. Your paper will have instructions on how to print.
Once you've printed your decals let the ink dry and you can use them like any manufactured waterslide decals.
If you want colored decals your best bet is to order them. Bell decal is a good resource for this as well. They can make any image, photo, or original piece of artwork into a decal sheet. You can also order full sheets of color and cut out your own shapes. These decals are applied the same way as the iron decals, but fire much lower (generally between 022 and 017).
If you want to follow the rules and have a really clean decal you want to bisque and glaze the surface first. a white or clear surface is best if you don't want the color distorted. If you want to get crazy you can apply the decals to greenware or bisque ware without glaze. You can put glaze, sig, or underglaze over top of the decals once they are fired, I would suggest firing them first though.