im not sure about a coupe but i recon they would be the same there held on with 4 nuts inside the boot 2 left and 2 right,,, then under the arch theres the steel bumper clips that clip the bumper and arch together and then just a couple of screws in the plash guards and the bottom of the bumper

 

I had mine off last year to change parking sensor, iirc theres two hex bolts that you access from underneath the back bumper lying underneath the car, and bumper clips at each arch

 

My bumper is a little loose at the arches. Possibly a loose clip?

 

Yeah definitely. They sag over time anyway. I ordered up both parts for left and right side - 4 in total - not expensive for the big improvement it'll make.

EDIT: here's an image of parts (brackets/supports) numbers: 6 & 10