Motor Mount install for Subaru Impreza WRX/STi:

This how-to is based off a GR, but the concepts should be similar to a GD. I’m not sure how different the front subframe is or how it might interfere with the install.
8.) Remove front swaybar D-brackets by removing 12mm bolts and 12mm nuts (one each per side, shown below on driver side):

Then swing the entire swaybar down.

9.) Using your impact or breaker bar and the 14mm deep socket, remove the two 14mm nuts from the bottom of the engine mounts, circled in red.

10.) Now, using a jack with a block of wood, jack up the motor by the oil pan until the studs clear the subframe. I can tell you the first thing to bind will be the AC line that curves around the air pump on the driver’s side by the battery. You shouldn’t have to go that high though. It is good to have a spotter watching around in the engine bay just in case. If you have it on ramps, at first while jacking the car will rise as the suspension lifts while the weight of the engine is taken from it. Don’t be alarmed as it is not stuck (as long as you followed the above steps). Keep going and you will see the engine lift. Take a jack stand and block of wood and put it under the transmission oil pan to offer some protection from the jack falling.

Here is an optional alternative method to protecting yourself. You can take the nuts and screw them onto the shaft on the engine side. This way, if the jack fails, the stud and nut will catch the engine. This is only effective if you only remove one mount at a time (which you should do anyway).