Best advice I can give you is start at the relay/fuse box next to the battery.
Pop the bottom cover off and unplug 2 connectors. I think one is white and the other off white.
Unplug starter, coolant temp stuff, Cruise control, remove diagnostic port, and unplug distributor, ignitor and coil.
Unbolt fuel rail and the harness from the intake manifold.
From there it's stuff like pressure sensor, ac idle up, egr, tps, idle air control valve.
Then move over to the ac compressor, alternator , and radiator temp sensor. Peel all that back.
From there it's just the bulkhead connector and 2 grey connectors near the AC lines at the firewall. Inside the car need to unplug the ecu and it should come right out.
Search for the bgb on one of the Celica forums. I forget which one had pics of the layout.
Edit. Going from memory you have to remove the alternator to get to the piece that runs between the timing cover and valve cover. It's also bolts in 1 or 2 places.