Glute-ham raises either requires a glute-ham raise machine or some creativity. In this exercise, you position yourself on a glute-ham machine or Roman hyperextension bench face down with the backs of your ankles secured by a foot pad. The upper-to-mid part of your thighs should be resting on the rounded part of the bench. Lower your trunk until your trunk is as vertical (and upside down) as possible and your head is pointing directly at the floor. Start the exercise by extending at the back and hip until your upper body is perpendicular with the floor. From this point, flex at the knees until your upperbody becomes nearly vertical. Then lower your body and repeat.
Yogis require a partner or some object to secure the backs of your legs. You start in a kneeling position with your ankles secured by a partner or some immovable object. Then lower your upper body to the floor while maintaining a straight line from your shoulders and knees. Go far as you can and then raise yourself back up.
Flags also require an immovable object or partner. Lying on your back, with your legs fully extended, put your hands over your shoulders and grab the stationary object or your partner's ankles. Then without flexing at the knees or hips bring your legs, hips, and lower back off of the ground, return them to just slightly off of the ground as one unit, hold for a second with your weight completely supported on your shoulders, lower and repeat. If you can't do this as described, start with an L-up–> Hip raise and then lower slowly in the flag position.