Stand in front of a flat bench about a foot away from it. Have a spotter help you get in position by helping you balance yourself. With the loaded barbell on your back raise one foot slowly and place it on the bench behind you. In this position, take your time in getting in perfect balance before you start your initial squat. This is your starting position.
Squat down slowly all the way until your back leg's knee is a few inches off the floor. Then, push back up as strong as you can until you are back to the starting position.
Finally, repeat the entire movement for the desired or prescribed number of repetitions and then switch sides and do the exercise with your other leg.
Perform this exercise in a smooth and fluid motion
You MUST have strong legs in the first place to even attempt this exercise
Don't load the barbell right away; do one warm-up set with the empty barbell
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or increase the weight.
This exercise is not for beginners. This is an exercise that requires a solid experience in doing squats as well as one-legged bodyweight squats. The balance aspect of this exercise is the most difficult component to master.
Your best approach is to start off with the (unloaded) barbell only and do several practice repetitions until you find the perfect equilibrium for optimal balance for this exercise. Once you are able to dip down with stability and proper balance you can load the bar with some weight.