Just put the heal of your leg on a chair, log, stump, step or anything, the more creative, the better. Keeping your leg straight lean forward and reach to touch your toes with both hands, bending at your hips. You should feel the stretch the entire length of the underside of your leg, if you don't stop, readjust and try again. This is stretching the semitendinous and semimembranosus muscles in your upper leg and the soleus and gastrocnemius in your lower legh.
Don't worry if you can't quite reach your toes, i can't either but persistance pays off and you will get more flexible the more you practice the exercise.
Don't ever under estimate the importance of stretching and the benefits it holds.
The Standing leg-up hamstring stretch can help with lower back muscle strain, lower back ligament sprain, hamstring strain and calf strain.
Always remember that while stretching may be recommended to aid recovery out of injury, you should always be very careful when stretching through rehabilitation. Your soft tissues are likely to be more vulnerable to re-injury if you stretch too much or too hard during this time. Always check with your Health Care Professional who is guiding you through your recovery to make sure it is appropriate to start stretching, and please - ALWAYS FOLLOW THE RULES FOR SAFE STRETCHING (The first posting on this stretching blog).
Image taken from "Lower Body Stretches" wall chart by B. Walker.