Holding dumbbells at your sides, you step up onto a bench with one leg and drive through that heel to bring the trailing leg up, then step back down under control. Choosing a bench height where your knee stays around 90 degrees keeps the tension on the quad and glute rather than the lower back.
Log your dumbbell single leg step up on bench sets in 1RM.fit and watch your estimated 1RM update automatically as you get stronger.