Doing this in front of a mirror is helpful to monitor form.
Start with a firm foundation on one leg (weight evenly distributed over forefoot and rearfoot). Lift other leg up.
Pushing the hips backward (hinging from the hips), bend slight at the knee to go into a squat.
Go as far as you can without breaking form. Proper form means you move straight down and up (as in a double leg squat) without letting your knee or hips dive in or out and without losing balance or foot contact.
Think quality! Only perform as many single leg squats as you can with perfect form. Start with one and progress from there. Balance drills and hip/glute strengthening with other exercises will help improve your ability to do single leg squats.
Running is effectively a series of alternating single leg squats. Improve your coordination and strength here and you will become a better runner!