Once you have learned your subject, next comes observation. Your first option is reviewing the many introduction books on the market available with basics portrait drawing techniques. These techniques will give you the basic fundamentals on body proportion and colour, so you can have a clear understanding of what you are observing, but don't rely on this alone.
Train yourself to observe
It's not enough to know the fundamentals. You must now learn to paint what you see in all the possible perspectives, colours, shades keeping the fundamentals in the back of your mind. Understanding and observing should come together as one. To train yourself to observe... keep painting. As you continue to paint your observation will improve. You will learn to see with each artwork you create.
Research the arts
Look at other artworks... visit galleries. Read about the history of art... find similarities with today's artists and yesterday's artists. There is so much to see and experience.
Take your time
I don't believe it's a good idea to rush an art piece. Take your time, checking out the details, composition, and colours to choose. Walk away from your art from time to time. Look at it the next day or two. You' ll find that you will see things differently and a few adjustments are in order. Step back occasionally as you are painting and take a good look. Put your painting in front of a mirror and look at the reflection. Place your work upside down and observe closely... you'll be surprise how the mind works.
Continue to learn by creating...