Drag it on the car to move it. Shift+drag or Shift+wheel resizes, Alt+drag rotates.
Arrow keys nudge · Del removes · Ctrl+D duplicates
Import a car to start.
Paint
A transparent PNG of the panels and their edges, at the skin's own size, to draw over in Photoshop or Gimp. For a shaded sheet you can paint on here, start from "Get a template" instead.
Painted it already? Hand it back here and you get the finished folder, with the DDS the game reads, ready to drop into content/cars.
Patterns
Decals
Click a sticker on the car to select it, then drag it anywhere. Shift+drag or Shift+wheel resizes, Alt+drag rotates, arrow keys nudge, Delete removes.
Text & number
Every text and number lands as a sticker you can drag, resize and rotate straight on the car.
Styles
Import a car to see every skin it holds.