How to practice at beginner–level, and how to teach beginners if yo're a teacher
Make sure the beginner is holding the foot properly; palm out, thumb back, elbow in. About half of the time, beginners will have a wrong grip. However, for someone with very tight hamstrings, knee problems, or overweight and with short arms, I'd let them grab overhand with a 'wrong' grip.
Don't kick back too much, or you won't be able to lower down. 'Unfreeze' the shoulder and let it go back. This way, the leg can kick up behind you, away from the hip.
Assuming it's the right foot that you're holding, you need to turn your right shoulder back (in the mirror, it will be now behind your head), and your right knee will be behind your body, so you won't be able to see either one in the front mirror. Keep your left arm strong and fully extended toward the front mirror, wrist straight and palm flat.
Work on balance and breath. If you fall, begin once again from the Mountain Pose, Tadasana. Usually, beginners lose their balance until they learn to: 1. Set the eyes straight ahead and to one place, drishti. 2. Slow down their thinking or movements.
Intermediate / Advanced practice
This pose is 40% back bending, a 40% standing leg hamstring stretch, and a 20% shoulder and hip–joint release. The key to the pose is judging how high to kick back at the beginning. If your standing leg is not flexible, you can't kick too high because of the limitation in the hamstring. So kick back just enough to lower the abdomen down parallel to the floor.
The more flexibility in the standing leg, the higher you'd want to kick at the beginning. At the advanced level, you will start by kicking super–high in order to release the back hip. Try to find your starting point within the first five to ten seconds of the posture, leaving you enough stamina to kick your leg up.
You need to kick up high right away, but slowly, in order to release the back hip—right at the beginning of the pose. This is important.
At your level, you should have the hamstring flexibility to lower down to where your abdomen is parallel to the floor, your chest is a little higher than your belly, and your arm and shoulder are a little higher than your chest. This completes the back bend. Get to it right away, don't waste too much energy getting to your starting position—start kicking. Hold right off your ankle toward the shin (maybe the pinky finger on the ankle, the other fingers around the shin.)
Look in the front mirror and see that the bottom of your upper foot is in a vertical position, flush to the mirror. If your upper foot is pointing inward or outward that indicates that your hip is not in the right position. Your knee should be directly below your foot towards the floor.
Do full splits, hanumanasana, down on the mat to stretch the hamstring of the standing leg between sets. This will help lock the kicking leg out. (Just don't do that in a bikram class, the instructor might throw you out of the room.) If you're not yet able to straighten the upper leg at the end of the pose, take a peek at the side mirror. This will give you a better visual. You're probably closer than you think. Then just lock out the kicking leg, yeah?