sit on the floor with your legs out straight in front of you. put your right foot over your left leg and place it next to your left knee, so that your right knee is almost fully bent.
hook your left arm around your right knee so your knee sits inside your left elbow. grab yhour left hand with your right.
keeping your right foot on the floor, pull your right knee towards your left shoulder and hold it for 60 seconds. then, dont release the pressure but tighten it as much as you can and hold it for 30. tighten again and hold for 30.
do this several times a day as its a thick old tendon you are trying to stretch.
this isnt specifically for sciatica as such, its more commonly used for certain types of knee pain but it helped a lot with my sciatica.