How do you rig crab rings for crabbing from a boat?

I use to do this all of the time at Bayou Bienvenue before they added the dam in the MRGO and ruined it.

I like to use empty 20oz soda bottles, like Sprite, that have a concave shape in the middle of the bottle, dish soap bottles also work well. The shape allows a place to wind your string such that it won't slip or unwind. I usually rig with about 20-30' of "dipped" or treated twine tied to the pull loop of the top of the net so you have plenty length to fish many depths. The concave shape also helps so you don't have too much line out when you pick them up. When you first put them out, drop the net until you hit bottom, give yourself 2-3' of slack and put a half hitch over the float and drop it.
Pro tips, make sure the weight of your nets won't sink your float if it gets too deep. Also, good, solid nets with a heavy ring will stay put in pretty heavy current as long as there's enough slack to keep them firmly on the bottom. Things like waves, heavy current, and wind should be accounted for with extra rope. Always pull the nets/ traps up running into the predominant force be it current or wind. If current and wind are heavy and perpendicular to each other, prepare yourself to have your patience tested and lose crabs...