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I have rappelled quite a few times. I have also done commando rappel on shorter patches (up to 100 ft or so). The pic of a cliff that I have attached below is Kokankada. The height to rappel will be around 1800ft into 4 stages. Out of that, probably a 800 ft will be pure overhang. I have rappelled here before, now I want to try a commando rappel at this place.
My question is: How are we supposed to rappel on such a large overhanging cliff? What are do's and don't's?
Rappelling is dangerous enough and doing this from very high heights is something to be taken extremely serious (unlike the video link you’ve given). All of the stuff that Kai K. has recommended I would assume you already knows without being told. In my mind, the piece about commando rappelling (very serious climbers do not do this) seems dangerous. I have been climbing in Yosemite seriously for over a decade and ascended all of the major walls there, so my experience is deep. In all those years, I have never met climber(s) that would do this on a cliff this size.
Since I have done this type of repelling before, I see additional points to stress:
Of course, the best practice in all climb excursions is being able to self-rescue yourself. Good luck to you.
Please make sure you know what you are doing before you attempt this feat. Just because you have successfully rappeled there before, does not automatically mean you were/are qualified to do so.
Very overhanging cliffs can't be rappelled in the traditional sense. "Bouncing," like another answer here suggests, does not work on steep overhangs. You have to down-climb, and if that is too difficult, you have to down-aid.
On multi-pitch rappel routes, it is not enough to know how to ascend the rope. You have to know how to actually climb back up!
Aid climbing is a skill that takes years to develop, is very exhausting and takes a lot of time if you are a novice. Trust me, I have underestimated my aid-climbing skills before. For steep overhangs you will need to bring actual climbing gear (cams, nuts, aid-ladders, daisy chains, and possibly even hooks, pitons, copper heads or a bolt kit). If your route finding skills are lacking you can get yourself into serious trouble, even more so as if you had tried to climb the cliff.
Make sure you know how to pendulum and tension traverse.
I don't mean to sound insolent, but from your vague terminology it sounds like you are not a seasoned aid climber. My advice is: Don't do anything you can't "undo:"
If you rappel for several overhanging pitches and realize that the rappel anchor is missing, or that you are off route, you will have to climb back up, since you have pulled your ropes.
There are multiple accident reports about this kind of feat:
1) Stranded—Unable to Locate Rappel Anchor, California, Yosemite Valley, El Capitan Shows that even seasoned climbers with aid experience might get into trounble.
2) Rappellers Rescued off of El Capitan An example of how exhausting getting off route might become.
3) Rescue of Climber Injured in Rappelling Accident Stresses how important route finding is.
These were all seasoned climbers!
Edit:
I just reread your post:
The height to rappel will be around 1800ft into 4 stages
Regular climbing ropes are 200ft, which is the maximum distance you can rappel with two ropes. If by "stages" you mean pitches, this makes for at least 9 pitches. I would call that a big wall. You might spend days trying to climb back up if you get into trouble.
