Thanks for downloading BetterDecals.

Installation guide (just in case you didn't read the first README):
1 - Move both Decals and Mods folders into your game's folder (where My_Garage.exe is located).
2 - Add some images to the folder Decals (.png / .jpg / .bmp, more formats probably work but they are not tested).
3 - Buy a decals box / use the one that you have and enjoy

Also, as a quick note, the keys Z and X are used for making the sticker bigger or smaller (Z -> Smaller / X -> Bigger)