Basically, you're just stepping up onto a platform with weight on your back. I use an olympic bar and an open squat rack. I put my platform in front of the rack - I use aerobic steps for my platform - and unrack the bar by walking forward, toward the platform. You place one foot firmly on the platform and stand up without leaning forward too much or pushing off too much with the foot on the ground. Once you're standing on the platform, you step back down and repeat. I do one leg at a time, so the same foot stays on the platform during all my reps with that one leg, then I switch feet and perform reps on the other leg. When you're finished with both legs, you walk backward into the rack to re-rack the bar.
I'm tall, so I'm using seven aerobic steps for high step ups.