Why people get it wrong

The reason that most people do not effectively or properly stretch the lower leg is because they are not familiar with the anatomy and function of the calf muscles.  There are two (and for some people three) different muscles in the lower leg (gastrocnemius and soleus) that perform similar actions but under different circumstances.  One of these muscles works more when the leg is straight, and the other works more when the leg is bent.  In order to effectively stretch the calves, we have to do BOTH.

Keep it simple

If you want to properly stretch both, we can simply do the same action, but make one small tweak.  Start with the leg straight behind you and push your heel down.  You should feel a slight stretch in the middle of the calf itself; this is the gastroc muscle.  Then, to feel the stretch into the soleus, simply slide the back foot forward a few inches and bend the knee.  If you keep your heel flat, you should notice that the stretch moves lower into the area of the achilles tendon.

How long?

For these stretches, remember that the calf muscles and their associated tendons are VERY STRONG and can be very tight.  Spend a minimum of 90 seconds for each muscle and each side!