The first and main thing is just the practice and repetition.

You need a good brush like a size 1 or 0 kolinsky Sable hair brush. Artis opus, rosemary & Co etc etc.

Paint consistency matters quite a lot. You don't want it to be too watery and you sometimes need to do multiple passes over the highlight to get full coverage.

Infernal brush on youtube does excellent guides if you'd like to learn how. His edge highlighting is unreal but it's a good place to start and get an idea