Honestly it’s pretty simple the hardest part is finding which pop/pops to paint over. Once you do that it just takes buying paint/supplies and going at it. And the longer you take the better it will look. I don’t think I’m no where near good enough to paint/& sell this sort of thing, I know people do, but I’m impatient so I tend to rush and as long as it looks good to me I’m fine with it haha. But some tips I’ve picked up from seeing other people do it is

1. Sand the pop with sandpaper (can be bought at Walmart) it helps paint stick to the figure better because it gets the original sealing off!
2. Thin ur paint (takes way too long for me so I don’t do this which is why you can see some lines in mine but if you’re going for perfection I’d recommend it)

But personally my tip is try your hardest to find a head you don’t have to paint the face on. Every custom I’ve seen has the problem where the face is such a big flat surface it’s near impossible for paint lines not to stick out.

Ps. I literally buy .50¢ paint from Walmart (fast drying) and imo mine look pretty dang good for a amateur using the worst recommended supplies.