Fix a resistance band to a secure point, approximately hip height.
(The heavier the band, the easier it will make the exercise)
Hold the band with one hand, and balance one leg with extended the other leg in front of you, flex the toes to the shins to help engage the quad.
Bend the knee, hip & ankle simultaneously to descend into the pistol squat, using the band for assistance. Try to achieve full depth, with your glutes as close to your heel as possible in the bottom position. Pause before driving back up, again using the band for assistance.
Repeat for repetitions & then switch sides.
As you develop strength, switch to a lighter band & try to limit the amount it assists you in the movement.