Step-ups are a staple exercise for the lower body. Start off with your body weight and a low step, and work your way through the following progressions:
STEP HEIGHT
1. Low
2. Medium
3. High
DIRECTION OF MOVEMENT
1. Reverse
2. Side
3. Forward
LOADING IMPLEMENT
1. Dumbbell
2. Barbell (Back)
3. Barbell (Front)
If balance is an issue, use some support initially and wean off over time.
To maintain a more upright posture, elevate the step heel with a squat wedge. Iron Bull Strength makes the best one on the market.
For more details, check out Step-Ups Develop Muscular Legs.