This tip on the single leg deadlift comes from Gray Cook and the Functional Movement Screen people - and it really helps people learn the proper form for the SLDL.
Take a broomstick and place it behind your back. If your Right foot is staying on the ground (the working side)- your left hand will be holding the stick (palm towards you) in the curve of your neck. Your right hand will be holding the stick (palm away) in the curve of your lower back. The stick will be touching the back of your head, your thoracic spine, and your tail bone. Now bend the knee to about 20 degrees and keep it there and perform the single leg deadlift and keep the stick in contact with all three points of the body.
The pre-set of the knee to 20 degrees and keeping the stick in contact with the body will help you in hinging from the hip, maintaining the arch in your lower back and the proper form for the exercise. Make sure the knee does not "bow or cave" in but stays in alignment.
The SLDL will help teach the hip hinge, strengthen the ankle, improve "balance" (a blog for another day but balance is poorly understood and even more poorly applied), and hits the glutes and hamstrings very effectively.
Weighted SLDL can be performed holding a weight in the contra-lateral hand (Right foot on ground - weight in left hand), weight in both hands, and suitcase style in the same hand as working leg (right foot - right hand). But once you add weight do not lose the form you learned using the broomstick!