Summarize:

Get into a comfortable position on the ground. Extend the leg you want to stretch out in front of you. Bend the knee slightly upward. Keep your other leg out in front, or pulled up closer to youwhatever's most comfortable. Reach forward with both of your hands and lightly grasp your foot just below your toes. Gently pull back on your foot so that you have to flex it as much as you can toward your body. Your toes should point back toward your body, but don't pull so much that it hurts. You can also use an exercise band or towel wrapped around the ball of your foot instead of your hands. You should feel your calf stretching, but it should not be painful. Hold the position for twenty seconds, making sure to breathe while you do the stretch. Return to the original position when you're done, and repeat with the other leg. Repeat two or three times for each leg. Isometric exercises are based on resisting forces. To do an isometric calf stretch, start in the same position as a normal seated calf stretch. When you pull back on the ball of your foot, use your calf muscles to try and straighten your leg, but don't actually move it. The force of your hands should be enough to keep your leg from straightening. Don't push or pull so hard that it hurts.
Sit on the ground with your legs outstretched. Pull on the ball of your foot. Hold the position. Try an isometric variation.