This does take some time. I use a 3 1/2" hole saw, but a router will work too. The drill you see has a "fixture" I built that keeps it square with the floor. A noraml drill works, just take you time

First I locate a existing frame member, as you can not put the backing plate on a frame member. You can see the screws used to hold the floor down in you trailer. Use those to locate the frame underneath. If you must install on the frame members, I use a backing plate, but use only two holes, or a piece of 3 x 3 x .125 angle iron, again, long enough to spread the load out.  If you do mount to a cross memeber, alway install so the two screws furthest away from the pulling direction are in the frame. (the straps will have a tendancy to "peel" the d ring out of the floor)

I cut a 3 1/2" hole, all the way through the floor, with plywood floors. With 1.5" floors, you can stop about 3/4" through, and pop out the wood plug. The D ring will cover the hole anyway.

I then position the D ring over the hole, facing the direction I want it to go (the holes are not a square pattern!) and drill a 3/8" hole through each bolt hole, all the way through. I use 5/16" Carriage bolts and nuts.

Then, I have someone crawl under the trailer, and put the backing plates and nuts up and tighten.
 
 

Repeat as Necessary!