Stand on a small step (around 3-5" high) and lunge forward. Drive your front knee forward and keep your back leg relatively straight. Push your front leg into the floor until your back upright on the step. Repeat.
*These should look more like a split squat (rather than a 90/90 lunge)
**These can be done alternating, or done all reps on the same side before switching to the other. See your program for your specific prescription.