Stand up against a wall, wood fence, or tall tree. Walk your feet out and bend your knees so you are in a sitting position, as if you were sitting in an invisible chair. Press your back into your support and hold the position for indicated time.
If you have a hard time holding, use other exercises to complement your wall-sit to occupy your mind and distract you. Practice bicep curls with light weights, punches in front of you, or arm circles.