Though typically performed with just bodyweight, don't underestimate this exercise, especially as a primer or finisher for other quad exercises.
The Peterson step-up encourages forward tracking of the knee over the toe, along with a ball-of-the-foot contact point of the working leg, both of which encourage quad activation, with emphasis closer to the distal (knee) end of the thigh.
Avoid pushing off the ground with the non-working leg. Let the working leg carry the entire load. Be sure to stand up tall at the finish position. Remember, the quads are knee extensors. To get the VMO to do the most work, squeeze the knee tight at the top.
Once the reps become easy, increase the range of motion with a higher step rather than add weight, but be vigilant about your knee tracking. A good rep should show no compensatory action, such as the knee falling inwards towards the midline of the body. The knee should point where the toe points at all times.