If you've been training full pistol squats already, you might find these progressions easy. (neatstrength.com)
Now if you feel strong, you may want to attempt to complete a few full pistol squats on your own. (fitbodyhq.com)
Continue to work on your single leg assisted squats adding a bit more depth as you feel stronger, until you are able to perform a deep squat fully on one leg before moving on to a full pistol squat. (thebettyrocker.com)