I did this job a while back and wasnt too bad. Be careful about the foam base which contains the bolsters. Pre 90 spec gtis have a smaller base than the 90 spec and these are not interchangable.
The sides/bolsters are part of the base foam.
To replace, remove the seat from the car. Undo the necessary bits to seperate the back of the seat from the base.
On the base you have to remove the sides that allow you to get the fabric off. These were held in with thread lock on mine and I had to use an impact driver to loosen them. (Mine was a 3 door, not sure about the 5 door?)
Small bits of wire hole the fabric and foam to the frame, its tucked in under a "lip" on the frame base. When you undo this the fabric and foam comes away from the base.
You can then, with a bit of difficulty remove the fabric from the foam. It is held onto strengthening bars(?) that run on the inner part of the bolster. It may easier to cut these than try and undo them.
When I read this back, its not that easy to follow

If you take the seat out of the car and start to seperate it, it should become clearer. I should have taken pics at the time :embarassed:
Hope this helps