I've painted 1 shell in my life, so that makes me an expert
Clean shell first - very important!
Apply liquid mask.
With a sharp knife cut your pattern. Firm, but not hard pressure.
Remove masked sections that require paint.
Paint!
Remember - some paints will change colour slightly depending which colour is applied on top of it.
Here is my one and only shell painted using Tamiya rattle cans:
Flames are paint, the rest is stickers