Well, I'm not sure where you can learn the techniques.
We use paint instead of decals quite a bit on collectables. I'm not sure if it pays off because it's really labor intensive. We bought a plotter (vinyl cutter) and the software for it. Then learned how to use it from the manual. We create the artwork in the plotting program keeping in mind that the finished product will be masks. Multiple copies are then cut into masking material for each color to be sprayed. It helps to use an indexing mark or two outside the artwork. That way you can lay each mask in exactly the same location so the colors will be in the correct spot. Here's a couple of examples, these are all paint: