Your first move is to step back with your left foot so that you are sideways to the net, and then move backward quickly with a side-step or cross step. As you do this, draw the racquet back across the left side of your body with your left hand and make a grip change to the backhand.  Keep your left hand on the racquet to stabilize the backswing and help maintain your sideways alignment with the net. 

As you near the ball, raise your hitting elbow to about eye level.  Your forearm should be tilted down slightly from the horizontal.  Allow your racquet to point almost straight down.  Think of this racquet position as about 7 0'clock on an imaginary clock. As you reach your final position, your right shoulder and elbow can help you line up the ball.

Your shoulder, as it rotates upward and back toward the net, initiates the forward swing. As this starts, your forearm snaps rapidly upward and forward.  This in turn accelerates your wrist and racquet up to a 12 o'clock position and then forward toward the target.  At contact your arm, wrist, and racquet are straight and acting as a single unit. Your wrist should be firm throughout the stroke, just as it is on a serve or groundstroke.

Impact with the ball will usually be above and in front of your hitting shoulder. The racquet head itself can be tilted slightly forward at contact, though it will still be going upward on its swing path.  Coming from below the ball, your racquet will automatically generate topspin. Try not to hit down on the ball, or you'll drive it into the net. A slight pronation of your forearm before contact will angle the ball cross-court; maintaining the forearm and back of the wrist parallel to the net will send the ball down the line.

The position of your shoulders is critical on this shot.  You must be more than perpendicular to the net.  Given the physiology of the shoulder, you can only hit out effectively on a backhand overhead if you are somewhat facing the back fence with your upper body.  If your shoulders are just perpendicular to the net, your shot will veer off to your right (for a righty).  So, keep your back to the net.  Show your opponent your left shoulder blade.

Footwork and movement into position under the ball will depend on how deep and far over your head the ball has been hit, and where you believe you can get the most leverage. Usually only a few steps are involved, as the intent of your opponent in lobbing to the backhand side is usually to catch you off guard. There isn't much time for fancy footwork.

If you are able to catch up to the lob, your last step will be with your left foot and you can step forward or to the side and shift your weight on to your right foot. On shots almost behind you, you will usually jump off your left foot and land on your left foot.  If it is really deep, you will leap off of your left foot and land on your right foot (similar to a scissors kick). On these deep lobs, your body will be facing the back fence and the stroke is essentially hit with your back fully to the net.

A common error on these deep balls is trying to finesse the shot by using a good deal of wrist to snap the ball back into the court.  This usually won't work, because using the wrist reduces the amount forward momentum of the arm and shoulder, robbing your shot of power and depth.  Difficulty in timing a wrist snap will also lead to netted balls.

When do you use the backhand overhead?  I suggest you use it any time you can, realizing that if you don't try the overhead for the lob on the backhand side you are going to have to run it down and hit it as a groundstroke or a lob after it bounces.  This in itself is going to be difficult, and you will surely be on the defensive. 

The quality of the lob will determine whether you take it in the air or let it bounce.  If you can get your racquet on the lob by side-stepping or leaping, go for the overhead.  On a deep lob not hit with much topspin, even if you fail to make contact your momentum will take you back toward the ball and you might catch up with it after the bounce.   I would rather see the attempt than the omission on a ball that lands only a few feet behind the player; allowing it to bounce results in a weak defensive position.

You will get many opportunities to use the backhand overhead in doubles, especially when you are the net person in the deuce court.  When the returner lobs one to your backhand, hit the ball either straight back to the returner or down the middle between the two opponents.  If the lob is short, you can be more aggressive and hit at the net player's feet or down the middle.  Don't try the risky sharp cross court shot to the net player's left.  Take the lob away from the returner by showing that you have a solid backhand overhead, and you can start crowding the middle for more poaches.

Another time in doubles when you might use the shot is when your net partner on your left is lobbed and you cross over to intercept the ball.  If it is high and moving away from you, then your only hope might be a leaping backhand overhead.  Give it a try,  you'll look good in attempting it and you might just save the point.