I use a lot of the ones from the default folders. And I use masking, as well. I'll also use duplications to make an outline of a shape.
More than anything, I watched videos to learn some basics. I watched this video (below), specifically, for the Coca-Cola Porsches I did. It helped a lot on those cars and on lots of others. I slowed it down to 0.25 speed and paused it, a LOT, to see what decals he was using.
And I really don't make that many custom liveries. Just about all of mine are based on actual cars/liveries. I'm no that imaginative and have a difficult time taking something I can see in my mind and putting it into the editor.