Step sideways onto the box and own the climb.
Push through the heel of the foot on the box, don't bounce off the floor with the bottom leg.
Stepping onto a box from the side makes one quad lift your entire bodyweight through a big range of knee bend, with the glute of the same side steering the pelvis level as you rise. Working from the side rather than the front changes the hip angle and exposes any tendency for the knee to dive inward, which is exactly the weakness it trains away. You finish all reps on one leg before switching, so it also settles left-right strength differences that two-leg squatting hides.
Start around knee height or slightly below. The test is whether you can rise without pushing off the floor leg, if you have to bounce off the bottom foot, the box is too tall for now.
Neither is better, they bias differently. The frontal step up is the bread-and-butter strength move, the lateral version adds a side-on hip angle that challenges knee tracking and hip stability more. Rotating both keeps single-leg training complete.
Slow the first inch. Pull the bottom foot's toes up so you can't push through it, and think about driving the box away through your top heel. If you still can't rise cleanly, lower the box.
