For portraits, you have an extra layer of difficulty compared to any other subject: the likeness to the real person. To make sure you get that right, keep measuring and never get tired of comparing.
Compare sizes, angles, colors, and especially tone and intensity. Having the correct darks and lights, and correctly positioning bright and dull colors, create the portrait.
Start by establishing the main shapes, make sure the proportions and the forms are correct, then move to the details.
Eyes, mouth, and nose are specific to each person. Make them resemblant and lively.