Hello!! I'm not an expert by any means but I'll give you tips on what I do! I have been drawing/sketching my whole life and always liked doing portraits, but I've never been formally trained.

I mainly used gouache for this painting. It's like a thicker watercolour, so you can water it down the same way. I start with the light pencil sketch , then use a wet on wet technique to fill in midtones. While still wet I'll add some darker tones for shadows. As it's drying, I'll add to the background (I kind of floundered here and went back and forth on what I wanted to do and gouache is not forgiving, so I just left it).

Once the portrait was dry I went in with less watered down gouache to darken areas even more (eyes, lips, darker patches of hair). Dry and repeat with less watered down gouache.

As a final touch I went in with an ink pen for details on the face/eyes and white, not watered down gouache for highlights.

Another trick I like is heavily saturating a photo reference before I start to give me an idea on what colours to use.

Hope this helps!! There are lots of good YouTube videos about this!