Now I've not seen the '97 Tercel front speakers, but usually speakers are mounted on some sort of plastic baffle which in turn is mounted on the door panel.
I like to leave the baffle with the stock speaker so it can be put back easily when i go to sell the car.
So then you can:
1. Fit an aftermarket speaker in the hole left by removing the original speak and baffle, mark and drill holes in the panel that conform to the new speaker, and use short metal screw to mount. Check for window clearance as new speakers usually have larger magnets than stock. If you have sound dampner its a good idea to apply a layer over the panel area before the speaker is mounted.
2. Make a new baffle out of mdf. Use the plastic one as a template for the outside dimensions, and the new speaker for the hole size. Mount the new baffle using the existing holes that the plastic baffle used. Just be sure about your clearance to the inside door panel if the mdf brings it out more than the stock baffle.
Replacing stock speakers is usually more difficult than just popping out the old ones and dropping in the new.
Good Luck
Gene