It sounds easy enough but once you get into it it ends up costing more and more money. I did the swap on my WRX after blowing up the auto.

Start out by buying a new starter, clutch hardlines and softlines, master/slave cylinder, interior trim pieces, ECU, crossmembers, clutch and brake pedals, possibly new ABS hardware. I ended up swapping out the entire bulkhead wiring harness so that I would be able to retain ABS and cruise control as well as be able to pass the car off as a 100% mechanically and electronically 5MT car in case of resale.

Electrically there is a lot of splicing that would need to be done in order to at least get the car to run right. I didnt want to drive around in a car that was just spliced and hacked together, I wanted a completely OEM 5MT WRX after the swap.

Unless you've got a parts car I'd just sell the car and get a 5MT, it WILL be much cheaper in the long run and then you wont have to worry about your car being skewed in the DOTs eyes when trying to resell it or register it.

What year Celica? OBDII cars have so many electric connections to the transmissions that you will get all kinds of CELs after the swap.