Since everything, including the sleeves for the bolts, is such a tight fit you'll need someway to line this up.
Use one of the screwdrivers through the hole for the bolt to get things lined up for one side. Do this by wiggling or levering as you need.
Slide the bolt into the hole. When inserting the bolt do it just like the factory and install from the axle side.
Repeat this for the other side of the mount.
Jack up the rear diff until the holes on the diff line up with the holes on the mount. Try and avoid using the screwdriver thing if you can since the stud holes are aluminum.
It'll take some wiggling and jacking up and down, but you'll get it.
Take the studs and start to thread them in.
Thread in all the way, and tighten down as best you can. Your socket with the swivel can help here.
Now, using your open ended wrench to hold the original nut, back off the extra nut for the bolts.
Torque spec on these is 51.6 lbs, but it can be a bit hard to get a torque wrench on there so you may have to do it by feel.
Take those nuts and thread them onto the bolts. Torque for these is also 51.6 lbs (from the service manual).
On the STi be sure to reconnect the ground and rear diff temp sensor.
Safely get all four wheels back on the ground, and go for a spin.
After that make yourself a sandwich since that's all you've been thinking about since you pushed the bushings in.