Wooww ^^
Thanks all the kinds words and very happy that the step by step look usefull !

@ Tab : Alex said almost all.

It's a special paper, one side normal, one side rough. The title is printed in a mirror effect on the rough side of the sheet. In this way we can clearly read the title on the smooth side.
as the letter are write in white on the computer, it appear in transparency as printer don't print white !

So after you just airbrush with the colour you want on rought side, here gold, and the transparency letter become gold on the good side .