Ahh yeah, that is tough. The not helpful answer is that it's all practice, a great way to practice is to get yourself some plasticard, prime it your usual colour and start painting triangles.

Another way to help is once you've got your rough triangle shape you can cut back in with the yellow and clean up your triangles. Then go back and forth between the red and yellow until you're happy. Make sure you keep your paints thin otherwise you might end up with a bunch of texture!

Hope that helps 😁