Cut off the old belt with a knife. Inspect for possible causes of failure, such as misaligned pulleys, rubber buildup in the grooves or a worn-out tensioner spring. If everything checks out, install the new belt by unloading the idler pulley and slipping it into place. (Check the owner’s manual; the belt routing can be complex.)
4a. Learn that you should figure out the route of the belt before cutting off the old one because the belt's routing can be complex.
good point!
4b. Buy a car with a good little sticker or dataplate diagram in the engine compartment that shows the belt path. When doing routine maintenance, ensure that it remains clean and legible.
Last time I replaced a fan belt, I pulled back on the idler pulley as far as it would go, but still couldn't slip the new belt on. Finally used a screwdriver as a wedge to pop it over the top of the pulley, but this seemed like a bad idea at the time. Any suggestions to get that new, tight belt installed?