You can buy blank decal sheets in either clear or white, and for either Laser printer or Ink Jet printer. Make sure you get the right one for your printer otherwise it won't work that well.
You only need the white sheet, if you have white in the marking that you are trying to reproduce, because regular laser and ink jet printers will not print white (that is where the ALPS printers come in, but printers, parts and ink are getting scarce), otherwise you can get away with using the clear sheet.
If using an ink jet, the ink is water soluble, so the ink has to be sealed to the decal sheet using a clear lacquer or enamel spray. The stuff that comes in the Testors kit is just a relabeled can of Testors Lacquer Gloss Clear.
One thing about making your own decals, because it is sealed with a clear coat, the whole sheet is basically one big decal, so any decal you print has to be cut out very carefully and as close to the decal edge as you can get, otherwise you risk having alot of clear barrier around the decal.
I have done a few myself, mostly just license plates for car kits I have built as I can custom make one using the Acme License plate maker, lots of new and old license plates from all over the US and Canada.