Other people have done this differently, this is just the way I did mine. Read the forum, ask questions and then build it your way.
I used 3/8" x 4" Carriage bolts. If you look closely you can see the head of some of the Carriage bolts. I used 4 on each side and two on the ends.
First I built the floor frame and set it on the trailer.
Then I marked the location of where the carriage bolts would go. Using a wood bit slightly larger than the diameter of the carriage boilt head I drilled a recesseed hole about a 1/8" deep where each carriage bolt would go.
Then using the center of the holes I made with the wood bit I drilled down through the 2x4 with a 3/8" bit into the trailer frame. You will need a bit at least 4" long to go through the 2x4 and trailer frame.
I then removed the floor frame and painted the underside of the floor frame with this Fence Post tar like paint. After it dried I put the floor frame back on the trailer frame, secured it to the trailer frame with the carriage bolts using a lock washer and nut. Then I painted the rest of the floor frame with this fence post paint. Note, wear gloves it's messy. You could paint the entire floor frame, put it on the trailer frame and then drill your holes. The way I did it worked better for me.
