The Graffiti Pack allows you to use a spraycan to freehand draw anything you'd like. It also includes 8 additional spray decals. The spray can does not need to be changed to another skin in the repair bench.
It can be purchased here from the Rust item store.
Equip a Spray Can and hold reload to select a deal
Press and hold left mouse to spray the chosen decal
Press and hold reload to select Free Spray
Press and hold reload again to select a colour
Press and hold use (Default E) to choose a spray size
Press and hold left mouse to use free spray