Hi. Well, the main thing is to draw from references, not from your head. Find photos of the people you want to draw and study their features closely. Basically, learn how to draw. My high school art teacher gave this advice, "Draw what you see, not what you know." So being able to perceive, observe, and draw the details is very important. But this skill only comes with lots of practice
. Drawing is a physical activity, so the more you practice the better you will get.
It's that way with everyone. If I go for a long time without drawing anything, my skills begin to wane. It's not that I forgot how to draw, but it just takes time for everything to flow. Don't be afraid to start over if something just isn't working. It can be frustrating, but just take a step back, do something else, and then come back to the piece with fresh eyes.
I hope these tips help. Let me know if you need any more advice.