This short video sequence shows you how to use crutches effectively. You need to take your weight on your good leg while you advance both the crutches and the bad leg at the same time. Then you will be able to bear your weight through the crutches rather than on the bad leg.
In case you cannot see the video, here is a still sequence -
|Keep your weight on the good leg and advance the bad leg and the crutches at the same time.|
|Shift your weight over onto the crutches.|
|Then advance the good leg while your weight is being taken by the crutches.|