Allow front knee to come to a right angle - then push and come back to your starting position. Now do the other leg. 

Alternate so that each leg does 6 repetitions. Work up to 4 sets of 6 repetitions every other day. Take a 2 minute break between sets.

When this becomes easy try doing the lunge while holding light weights.

Start with Back Step Lunges

Front Step Lunges are a little harder!