The Decal project in the book describes how to use decals and about firing them. Here is some extra information not included in the book.
Storage of Decals
Decals are affected by humidity, which is reasonable considering that they are water soluable. Thus they normall come with a protective waxy cover, which needs to be removed before using, but should be left in place until use. But if stored improperly, these covers can adhere themselves to the decal and they are hard, if not impossible, to remove properly. One could try firing with this cover on them as they will burn out in the kiln, but this is taking a chance that the decal will look good. It's better to store them properly... just store your decals in an air-tight plastic bag and they should be fine.
Videos from AAE Glass
AAE Glass, in FL, is on my Resources list in the book and on-line - I buy many decals from this company. Tanya, the owner, has many excellent videos showing the use of her products. See this video on using the Illusion Transfers (type of decal). Remember that one type of decal is Dichroic Slide, which fires on enamel and looks like dichroic glass. You could fire one of these on, then fire on a layer of transparent clear and then fire on one of these Illusion Transfers. But of course there are many gorgeous enamel backgrounds you can use instead - check out the Enamel Application chapter in the book and what I've added on-line. Enjoy!
There are many places to have custom decals printed and of course you print your own with special paper that fired a sepia color; see Resources list. In addition, there is at least one company that sells a printer to make your own ceramic decals - Enduring Images. It's expensive, but in a grou environment it might be well worth the money.