Thanks so much! :)
The only tip I can really give is to learn patience. Maybe start just drawing parts of a face, like an eye or a mouth and taking the time to get all the detail in there and develop your technique before starting to move on to full portaits. Patience is definitely needed for realism :)
Unfortunately I cant be much help! i kind of wing it every time.
Generally though I fill in solid spaces of shading, the light and shade of the hair, then more defined strands over the top to make it look more realistic! I hope that makes sense haha :)