I take regular baseball or playing cards and cover them with whatever-since I usually collage it's not as important...but when you use good watercolour paper it's a good base to measure with. I like my cards to have a nice substantive base.
TO be honest, I have never had problems with decoupaging ink jet if it has dried for awhile=at least a few hours. Probably a good idea to use hair spray if you're really worried-but really, no problems on my part
To get an aged burnt effect-use an incense stick. It is easier to control than open flame. I also agree with Racky coo about inkpads. I also just pick an "aged" colour of paint ( like burnt umber) and rub it on. Practice makes perfect