My garage door is old and falling apart, plus the driveway at the threshold is uneven. There are big gaps at either end both along the side and bottom. What is the best way to seal these up? I can't afford a new door right now.
There's a problem. Rodents can get through tiny gaps. Doors generally need gaps around them.
Some ideas -
Remove whatever it is that is attracting rodents. Tidy up the floor. Put edible stuff into metal containers (I use metal bins with metal lids that I weigh down with heavy bricks).
Get a cat and fit a cat-flap. Cats love chasing rodents away.
Graft new wood on around the edges of the doors using glue, dowels, screws or whatever methods and hardware you have to hand. This means taking the doors off after scribing the required shape onto some the donor timber. If an enlarged door would scrape the ground when opening, you'd have to lower the ground or abandon this idea.
Have the door close against a fixed frame. You would probably need a rubber sealing strip or equivalent to close up gaps. Rodents chew through rubber.
I'd go at it with some blocking and weatherstripping and try to shim the bottom of the door to mate flush with the floor.
There's company that makes specialized rodent proofing materials, you may be able to get by with a door sweep that you put in and adjust it to sit flush to the floor when the door is down.