You can go 7 rafts high, although 6 is about the max. Drop the pins, seats, anything that sticks up above the frame. It can be lumpy, the raft above will conform to the lumps in the raft below, you just don't want any movement between the rafts or a situation where it can create a hole. Fill the bottom rafts with gear to keep the weight low and keeping adding rafts. You can lay oars & paddles under a raft on the floor running out the side or back between two rafts. Run the straps from the trailer through the D-rings on each boat, front and back. Tie the bow/stern line on the top raft to the trailer tongue. Tighten everything down tight. You want the webbing (line is too small) to pull down on the tubes. Then go.
Don't put anything that isn't tied down in the top raft, it will blow out.
In a pinch you can tie the first raft to the trailer and then tie each succeeding raft to the raft below it, using D-rings but these are wobbly and when turning the rafts will lean. (Never had one fall off, but came close a few times.) If you go with this approach tie all of the bow lines to the trailer tongue and I would tie the stern lines down also.