Setting up the hinging ramp on the back of the trailer can be a bit finicky, and is easier if you get a mate around to assist.
Run a full length pin through all the hinge tubes including the ramp. Align the two outer hinge tubes to the ends of the rear chassis cross member and adjust so that it is at least 6mm above the rear cross member (if using 3mm treadplate decking) and tack weld into position.
Allow around a half mm gap between all the hinge tubes (up to 1mm if galvanising) and tack weld into position.
Raise and lower the ramp and check for a smooth operating, if any tightness or binding occurs, break the tack of the offending tube and readjust.
For a trailer using timber or plywood decking, run a piece of 25 x 5 Flatbar across the end of the rear chassis cross member and mount the hinge tube to this. Raise the ramp so that the bottom ramp rail is at least 3mm above the height of the decking.
Before fitting the rear corner deck, drill a couple of holes for routing the lighting cable from the chassis rail to the rear corner support arm.
Clamp and stitch weld rear corner decks to the trailer (screw if using alloy)