What tools are necessary? How difficult is the job?

The Highlander should have one, continuous, Serpentine belt.
The belt routing diagram is on a sticker under the hood.

Loosen the belt tensioner with a wrench, take off the old belt, re-route and install the new belt, tighten the tensioner, and you are good to go.

Stop by your local repair facility to have the belt adjusted for proper tension. In most cases, you will not be charged for this service.