To do the warm-up stretch:

Lie on your back in the supine position and bring first one (bent) knee and then the other up towards your chest. Hug them either at the top of the shins or at the back of your thighs near the knee. Pull towards you.  Stay in this position for 5-30 seconds. Gently set one knee down, and then the other. 

To do it:

From supine position with both knees bent and feet on the groud, bring one knee up towards your chest. Take the knee that’s up over to one side.  To “get” the out hip, where the piriformis is located, think about aiming the knee towards the opposite shoulder.  The leg that is “standing” will likely be brought over as you do this.  You don’t need to correct for this—it’s normal. Stay in the stretch for 5 to 30 seconds. Gently return yourself to the start position. 

The distance you’ll move the leg will vary according to how tight your piriformis and other hip muscles are, as well as how much pain you experience. Always stay pain-free. Only go so far as you can without strain, soreness, or similar muscle-related distress.

That said, because you are stretching, there’s likely going to be a little leeway. Some muscle discomfort is expected or you won’t benefit from the stretch, but you shouldn’t go so deeply into the move that your sciatica acts up or you experience any type of nerve sensation.  

Nerve Sensations in a Piriformis Stretch

Because the sciatic nerve is located under the piriformis muscle you may get sensations that do not arise from muscle tissue. You need to be careful if you experience these feelings.

They’ll probably feel like an electrical sensation of some sort that goes down one leg—pins and needles, shock, tingling, or even burning. You may also experience weakness and/or numbness in one leg.

In ​case of any, some, or all of these symptoms, back off the exercise and talk to your physical therapist or doctor about what you should do.

Try bringing both knees down to one side. Go only as far as you need to reach an “edge” where you feel something is happening in the muscle, but it’s not overwhelming or painful. Stay there between 5 and 30 seconds. Gently return your legs to the original “standing” position. 

To intensify further, you can progress to piriformis stretching for intermediates.