The main Meditation asana, ‘Siddha’ means adept or accomplished. So Siddhasana means Adept’s pose. For a Woman, the same pose is called SiddhaYoniasana; where ‘ Yoni’ means Womb.
This is the preferred pose for sitting for long durations in Meditation as the back remains straight and the heel is placed in the perineum thereby stimulating the physical trigger point of Mooladhar Chakra, which should cause the upward flow of Kundalini Shakti when Kundalini energy has been awakened.
Place the right heel on the perineum and the left heel above the right so that the ankle is on top of the ankle. If it is more comfortable to place the left heel below, you can do that.