For the Psoas stretch: come into a lunge, front foot should be out far enough that when you stretch forward the knee doesn't go out over the toe. Keep both hips facing forward tuck the pelvis slightly, push the hips forward, and raise the same arm and the leg that is back. Stretch up with the arm and slightly backwards and slightly to the side of the front leg.
For the Piriformis: either sitting on the ground or sitting in a chair, cross one ankle over the opposite knee, sit up nice and tall and try to push the knee down to be level with the ankle.
The Psoas Muscle and the Piriformis Muscle: Two big problems in hip and back Pain ….. relieved
Also check out this video for the psoas stretch: