With the left leg bent ad the foot tucked up against the perineum, take your right leg with both hands and, keeping it straight, bring it up toward the head.
At first there will be a tendency to bend both your leg and your back. Try to gradually straighten both out as you progress.
You can make it dynamically at first by exhaling as you bring the leg close to the body and inhaling as your release it to an arm’s length.
When this becomes comfortable, you can hold the leg there and breathe and relax for a while.
This will stretch all the muscles of your legs.