We just covered this in my enameling class.
The decals need to be printed with a laser toner that works. Some do, some don't. I know Delphi Glass recommends using a MICR laser toner cartridge with their decal paper. (Those are the kinds of cartridges that are used for printing checks. I think they have a higher iron content, but I don't know for sure on that.)
Like any water slide decals, do what the others have mentioned, soaking in water, then sliding the image onto the enameled surface. Let dry.
Our Paragon kilns have windows, so we put the piece into the kiln (at regular enameling temperature), watch for the flash (the plastic burning off), then time / watch it for about 50-60 seconds. It's usually 'done' by then -- decal still clear, but securely on the enamel.
Without the window, I'm not sure how to judge the timing on it, except maybe do a number of tests.
So many factors involved. I hope this helps a bit.