Hi Robert Philkill. Great question. I've attached a couple of videos that I think may help. While not only focusing on stretching the piriformis, you will also want to stretch your hamstrings. Also doing some routines to open your hips may be helpful.

This is always a good video too, as it covers some many areas: https://grokker.com/fitness/video/foam-roller-mobility/57dafebb9e105c6c5a704cd1