Load the barbell on a squat rack and unrack it and place it on your back resting on your traps and shoulders. Stand about a foot away from a flat bench and get ready for the initial step up. This is your starting position.
With one leg leading forward step up onto the bench and then place the other foot right next to the leading foot. Stand there on the flat bench for a fraction of a second and then step back down leading with the same foot that you used to step up.
Finally, repeat the entire movement sequence for the desired or prescribed number of repetitions.
Perform this exercise in a smooth and fluid motion
Maintain balance at all times
Keep your back straight
Don't do this exercise if you're a beginner; there are many other less complex leg exercises suitable for beginners
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, increase the weight or increase the height of the bench (improvise by placing weight plates under the flat bench).
This exercise is not for beginners. It requires some previous experience with exercises that require good balance. That being said, balance is the key aspect to this exercise. Even as an intermediate to advanced exerciser you will need to do a few practice reps with an empty bar to get your balance right. After you do this exercise for a while, balance will no longer be much of an issue.
Another worthwhile point to mention is that instead of using the bench to step up on, you can step up on a lower aerobics platform until you are ready to challenge yourself with the much higher bench. Thus, if you have problems with balancing yourself while using the bench as a platform, it's especially useful to start with a lower platform.