Which of these muscles? If you mean the largest:
1. Lie flat on your tummy, fully straight.
2. Pull leg out laterally about 15 degrees from normal.
3. Lift head and breasts off the floor, so you're resting on your elbows and abs.
4. Point fingers like you have high heels on.
5. Lift leg up vertically without moving the rest of the leg (knee/foot).
You should feel them isolate if you do it right. Imagine you have something between your buttcheeks, like >>128144 said.