Question: how to do ardha siddhasana?
Answer: Sit in Dandasana, fold the left leg and place the ankle in between the genitals and the rectum.
The ankle of the right leg should be kept on the upper portion of the genitals. ... 
The toes should rest between the thigh and the calf muscle.
The knees should rest on the ground. ... 
The spine should be erect.