Here are some key points:
Grip: I use a pronated grip
Breathing: Before I lift my legs so they are parallel to the ground, I take a deep breath in. I hold my breath for the majority of the rep, and if I need more air, I do a small exhale so I don't lose all of my internal pressure, and I inhale again.
Shoulder blades: I make sure that my shoulder remains pinned for the duration of the exercise. If I don't, my shoulder joint will be very unhappy with me.
Alignment: Throughout the entire set, I make sure that I keep my ribcage down, and do not hyperextend my lower back. I also make sure that my ribcage doesn't rotate and my spine doesn't twist. I achieve this by bracing my core, squeezing my glutes, and actively tucking my ribs towards my hips. I keep my neck in a neutral position.
Legs: I actively engage the muscles in my glutes and legs, as a rigid body is more stable and easier to support than a body that is disengaged and limp.
To note, if you do not engage your glutes or anterior core, either your legs will drop, your hip flexors will kick in, and/or your lower back will hyperextend, or your torso will rotate.
Do 3 sets per arm and aim to hold for 10-30 seconds.