On your side position your elbow under your shoulder.
Straighten your body and hold.
Hold for the desired amount of time.
Positioning elbow too far up.
Hips caving down.
Torso not being straight.
Starting on your side, position your legs on top of each other, then move your elbow to be underneath your shoulder and bent at 90 degrees.
Raise your torso up to have a straight body and hold. Tense your core to maintain a straight torso throughout. Repeat for the desired amount of time.
Avoid caving inwards, keep a straight back.