So basically, the whole mix, EXCEPT the drums, is being sent to the Rear Bus. And the compressor used on the Rear Bus must be dual-mono, to keep good stereo separation of the mix.
The quick and easy way to set this up, is to have your mix channels grouped into buses. Like a bus for all drums, a bus for vocals, etc. Typically, something you set up beforehand in a mixing template.
Going from here, you first add a new mix-channel that will be your Rear Bus. On this channel, you insert the compressor you are going to use for this. I use the Mace for this, as it is the only compressor RE (as far as I know) that is dual-mono capable. But also because it is a very versatile compressor, with loads of tweak-ability. You can also use a pair of any compressor, running each in mono. See the video above for the initial settings, but do experiment with the comp settings after setting all this up.
Finally, add a parallel channel to each group-bus, again EXCEPT the drum bus, and route these parallel channels to your new Rear Bus channel. I set the parallel channels at the same level as the buses they are linked to, but you can also experiment with these levels when everything is in action. Then, mix in the Rear Bus channel underneath the total mix.
Happy Rear Busing