Sit with the legs out straight. Pull the left foot into the groin. Place the right foot over the ankle of the left foot so that it rests near the thigh. Straighten the spine. This position can be done on the alternate side as well.
A delicate and effective variation is to assume Sukasana and then lift the heel of the foot near the groin. Bring it away from the body two or three inches. Arrange the foot on top so it rests directly on the calf with the ankle of the top foot about two inches up from the ankle of the bottom foot. In this pose make sure to press the lower spine forward. It will have a tendency to lose its natural, supporting curve.
If the first two postures are too strenuous for you then try this variation. Sit up with both legs straight. Put one foot under the opposite knee and then draw the extended foot under the other knee. Pull the spine up straight and press the lower spine slightly forward.
All of these poses require less flexibility and are easier on the knees than the Lotus Pose. The drawback is that you must be more conscious of keeping the lower spine slightly forward so the upper spine can stay straight.