There are two options in these plans for the rear of the ground loading trailer.
The first option, (details shown below), is a fixed ramp which extends back from the trailer.
The advantage of this style is that there is no walking to the rear of the trailer and fiddling around latching and de-latching ramps - its just lower the deck and roll on, roll off.
The disadvantage is that the license plate support can be a bit of a trip hazard, takes up access to a bit of potential deck space (if you want to get fancy, make if removable!) and while functional, is not the most atheistically pleasing to look at.
If your country/state regulations allow, the license plate could be fitted to the rear of the guard.
The other style is a hinging ramp and has the advantage of supporting the license plate and keeps the rear deck area clear when open. It also has the advantage of self adjusting to the type of ground the trailer is parked upon.
It is a bit more work to fabricate and marginally increases the windage of the trailer.
The ramp support plates need to be fitted to the rear cross member so that the ramp cladding (3mm Steel Treadplate or similar) is flush with the top of the decking material when fitted.
Fit an offcut of decking material to the trailer rear end, and with a piece of ramp cladding and a ramp support, find the best height for the ramp support to get the best result.
Mark the rear cross member for the correct position and height of the ramp supports and tack weld into position. Check for square and place some good strong tacks on both sides, top and bottom of the ramp supports before fully welding.
Tack weld the ramp cladding onto the ramp supports, and once fully tacked, stitch weld the cladding, staggering the welding to prevent any distortion.
Before fitting the rear corner deck, pre-drill a couple of holes in the chassis rails and rear corner deck supports for routing the light cables through once the trailer is completed.
Fit the rear corner plates and deck to the rear corner deck supports.
Before fabricating the license/number plate frame, check your local plate sizes to ensure correct fitting