1. Sit in a sturdy chair. 

2. Place the ankle of the leg you want to stretch over the opposite knee in a “figure 4” position. 

3. Lean forward until you feel a stretch in your buttock. Hold 10-15 seconds and slowly return.

Source: http://www.thera-bandacademy.com/tba-exercise/Piriformis-Stretch-in-Sitting---Older-Adult
Photo: http://highlandtraining.net/images/seated-hip-stretch1.jpg?crc=281552064