Siddhasana is actually a very specific pose described in yogic texts where the bottom heel is placed exactly in the center line (right under the genitals for men and pressing against them for women), but this requires fairly open hips to maintain comfortably and for common use in most western asana classes, we don’t really need to worry about that. Some might say the bottom foot MUST be placed near the perineum. Fine. It’s really up to you. Siddhasana and Padmasana are the two most commonly used postures for seated meditation in the yogic tradition.
Sit down on your mat or a blanket. bend one knee and place the heel near the center line. Bend the opposite knee and place the foot on top of the bottom ankle. The top foot rests comfortably across the bottom leg or (more traditional) is placed between the calf and thigh of the bottom leg. Make sure that you switch the foot that is placed first with each practice so that each side gets worked evenly in the long run. Extend the arms straight, placing the backs of the hands on the knees. Bring the thumb and index finger together in the Chin Mudra.
It is said that if you practice Siddhasana for long enough, you will gain supernatural powers (which could come in handy). From a purely physical perspective, it helps to open the hips and is said to help with stiffness in the knees and ankles.
If you have any questions, please leave a comment below. I’d love to hear from you or see you at one of my classes!