I hate to do this, but time to fire up the old search engines. Been documented many many many many many many many many many times. There are probably thousands of different methods and combinations.
Hints:
Your '87 GL-10 is an EA82 chassis.
Motor mounts are the same for all EJ engines (as far as how they bolt to the engine crossmember). "Somebody" spent an enormous amount of their life compiling an EJ swap manual over 10 years ago, while it focuses on the simpler SOHC EJ22, a DOHC turbo version will be very similar.
There have been STi versions of the WRX model for 26 years globally, and 16 years in the US. While the mechanical challenges of fitting it in your car are basically all the same (To date, they all use EJ series engines), there's a huge variation in these engines. If you want more specific answers, ask more specific questions.
Presumeably your GL10 is already turbo, if not, you'll need a turbo engine crossmember or custom exhaust.
Spend a couple dozen hours studying all the EJ-swapped EA82 cars, even the simple EJ18 and EJ22 ones (and of course, the turbo ones). Then spend another couple dozen hours studying turbo swaps into base model Imprezas and Legacies.