This is not for everyone but someone who wants to try something hard. Also if you are relatively tall and big, it may be more challenging. Use your discretion and train safely always!
Prerequisite for pistol squat
Certain level of flexibility is required. You want to stretch hamstrings. I worked on hamstring flexibility, static and active.
Example of static hamstring stretch
Assisted Pistol Squat
I learned this while doing gymnastics. If you want to learn something, you need to do the movement over and over. Assisted pistol squat makes you learn how the movement should be done. I used TRX as a demo but a dowel or wall are fine, too
Focusing on eccentric
What is eccentric? a muscle is eccentrically worked means that muscle is the prime mover of the movement and it is lengthening. Biceps curl for instance, going down phase is when biceps muscles are eccentrically trained. This method works because people are about 40% stronger on eccentric muscle action than concentric muscle actions counterpart.
For pistol squat, what your focus is down phase. Going down slow and letting the muscles work against gravity. The first attempt, you may not be able to go as low as where you want to be. But over time, you will be able to go lower and lower.
Going up with momentum
I will say this again. You need to do the movement if you want to learn it. After focusing on the eccentric phase, lying down, rolling slightly and standing up with some momentum with a single leg. This again will teach you how the movement should be done.