I take the original, whether it be Verlinden, Tamiya or any other manufacturer and put them in a Xerox machine to make colour copies.
This not because Im cheap but I do want them on thin paper. Next I cut them out roughly and glue them to plywood or plastic card of the appropiate thickness.
When the glue has dried a use a knife and a ruler to cut them out more precise. Usually one has to paint the borders. Heres one small vignette I did a couple of years ago, using this method.