I put one on my GS850G twenty years ago and it's not been off since (apart from when I replace tyres, of course).
I picked up four attachment points, two at the rearmost ends of the upper frame rails and two at the loops carrying the exhaust cans and passenger footpegs. Just carried them down and rearwards from there, to meet at a simple angle-iron bracket which the hitch bolts onto. The tow hitch is a perfectly normal 50mm ball, commonly used for trailers and caravans.
The trailer is a simple steel-framed box, 4ft long, 2ft wide and 2ft high. At the time I wasn't aware of the Golden Rule of nose length to axle width ratio, but ended up with it fairly close, by trial and error. The result was good, but not perfect - it's still a work in progress, but I was able to pull a full load at any speed without the trailer making its presence felt in any adverse way, either on the straights or bends.
I'll lengthen the nose by a foot this year, when I fit the new ball hook-on.
I made a swivel-hitch nose for the trailer, but it turned out to be less than useful, to be honest. I'm thinking of just scrapping that idea and going with a normal non-swivel hook-on as it gives all the swinging and tilting room I would need, and many people have found that a perfectly ordinary hitch is adequate for bike use.
I have another design for a swivel I might try first, though.
I'd thorougly recommend this site for a general feel of the subject and pick up some really good information.
http://forums.delphiforums.com/mctrailertowing/start