The pistol squat - also known as the one-legged squat - is an advanced movement that requires strength, balance, flexibility, and coordination. You should be able to flawlessly to an air squat before attempting a pistol squat.
Expect to feel this exercise in your glutes, hamstrings, calves, hip flexors, quads and core. Don't be worried if the quality of the squat feels different from one leg to the other. Doing single leg work is important for detecting and working on imbalances that may occur on different sides of the body.
Start with your feet hip to shoulder-width apart depending on your personal preference and anatomy. Fix your eyes forward and engage your core while you raise your arms in front of you and pick one foot up off the ground.
Bend the knee of the grounded leg and sink into a squat with control while the raised leg is straight out in front of you. At the bottom of your squat, your hip crease should be below the top of your knee. Your raised leg should be straight in front of you hovering above the ground.
Press through your grounded leg to come back to full extension. The raised leg should not touch the ground during this entire movement.
This is one rep. Complete another rep on the opposite side.
Again, the single-legged squat is an advanced movement. If you can't quite complete the movement just yet, don't fear - there are ways to modify the movement to work your way up to it.
You can do assisted pistol squats by holding onto a sturdy object (like a doorframe, squat rack post, couch, etc), TRX bands, or rings. Hold on to the object to assist you as you move down and up throughout the squat. Be sure to allow your lower body to do most of the work as you build up the strength and flexibility.
You can also do modified pistol squats to a box or chair. Decrease the height of said object to increase the difficulty.