Here are a few art classroom web-sites for you to study from.
I am about to do another portrait of my granddaughter . I first take a digital picture and download it to my computer. I do measurement changes first and then play with the color corrections. If I don't like the values in the portrait, I can change them in photoshop until I am happy. I am learnings all this in online classes I am studying. This is the picture I will be painting and also a quick drawing.
It took me 4 hours to get to this place in my drawing. (The bottom picture.) It should only take 45 minutes. That shows me I have a long way to go before calling myself a portrait artist. I am not talking about speed, but a combination of accuracy and speed. Taking that long time means I am not able to see all I need to see yet. I will see better as I practice more. Every inch of the face is important and I am not able to see what I need, to get it right all at once. But in time I will know what to look for without playing with it so much.