Get the rear of the car safely up in the air. Put the car in gear, and block the front tires just to be sure. As shown in the pic below jack up the car by putting the jack under the rear diff.
Place jackstands at the jackpoints just in front of the rear tires (there's a cutout in the sideskirt for this) and lower the car onto the jackstands, but do not remove the jack as you will need it later on.
Slide under the car and take a look at how the rear diff attaches to the mount.
There are two double ended studs toward the middle that attach the mount to the rear diff.
There are two bolts (one at each end) that go through the subframe and through the bushings in the mount
All nuts and bolts are 17mm.
Liberally spray the studs and nuts for the bolts with PB Blast or your equivilant, and drink your cup of coffee while waiting for it to soak in.
On the STi disconnect the rear diff temp sensor and ground by pressing on the tabs for the connectors and pulling them off