What are the fundamentals of a one-handed backhand? What grip should I use? How should I position, and what should the technique be like during the stroke?
I've always played with a OHBH. It seemed more natural to me, and I thought it looked cooler too. I naturally was able to slice the ball, but getting good topspin on a OHBH always eluded me until I heard someone say:
"Look through the racket when you make contact".
In other words, when you make contact with the ball, look at the ball through the string bed of the racket instead of looking at the ball in front of the racket. This can be done with an eastern or "extreme"-eastern backhand grip, index knuckle at bevel 1 or 2, respectively.
This will force you to hit a little out front, enabling you to follow through with your arms in an "open wing" position like Wawrinka.
If you're looking for a slice or backspin OHBH you kinda have to look at the ball from the front or side of the racket, as seen in the following image:
Slices and chips can be done with continental grip.
I think that's the best advice regarding the OHBH, anything more is harder to explain through text. Most of the time when I tell these things to people I hit with, things start to click afterward.
Also see this article: http://www.revolutionarytennis.com/federervisiontechnique.html
I am a one hand backhand player who was forced to use two hand grip for lessons back in the old days. I find that the technique is practically the same, the only difference is whether or not you use your non-dominant hand. The position of your dominant hand should be the the same in both one hand and two hand backhand. You should still turn, hit follow through in the same manner.
Of course if you do not know two handed backhand, everything I just said makes no sense, so in that case please let me know so I can explain it better.