Consult your owner's manual to get the proper part number for the drive belt needed for your particular lawn mower model and purchase it.
Turn your lawn mower on its side or put your riding mower up on blocks so you can get to the underside of your mower's deck.
Clean the underside of your deck. This can most easily be accomplished by carefully scraping the underside of the deck with a long handled flat blade screwdriver. Remove any pieces of drive belt that are still attached to the pulleys that are visible.
Locate the tensioner. This is a pulley that can move when it is loosened. Use your ratchet to loosen the bolt holding the tensioner in place. Allow the tensioner to move closer to the other pulleys.
Check your owner's manual for the proper way to position your drive belt over the pulleys. On most models the drive belt simply goes around the drive pulley, the tensioner and the pulley connected to the blades, but some models are slightly different.
Pull the tensioner tight until there is less than 1/2 inch of play in the drive belt when you push on it between pulleys. Tighten the bolt holding the tensioner in place once the belt is as tight as the owner's manual suggests.