An essential stretch for good hip muscle length and balance between thigh flexor and extensors.
1. Use the arm rest of a couch and bring your heel to your butt. Rest your ankle on the top of the couch as seen here.

2. Keep the knee pointing towards the floor. Relax and hold for 15-20 seconds.

3. Breathe deeply, in through your nose and out through your mouth.

4. Repeat for both legs.

If you need something higher, try the couch back  to rest your ankle on and follow the process above.

Give it a try.