I found a set of engine bearings (Main Bearing Set + Rod Bearing Set) laying around in my garage with a few other parts... anyone has a guide that explains how to change them? I have the Haynes book for celicas somewhere but i'm pretty sure they dont detail that much how to change them...

Haynes won't have it, since it's pretty much rebuilding the bottom half of your motor. And to do it properly (or at least more easily) it's nice to have the motor out of the car. No sense at all in doing it unless there's a specific issue with a bearing or you need to rebuild the whole motor.

x2.