Servicing the Derailleur Pulleys
Derailleur pulleys should be kept free of crud. Sometimes, a problem with the rear derailleur will be due to a bit of grass and mud wrapped around the base of a pulley. Clean the pulleys thoroughly, carefully checking the area between the cage and the pulley. A degreasing solution helps.
Derailleur pulleys can be damaged if the rear shifting is out of adjustment (the chain sits slightly off center, scraping the pulley), or due to mud- and sand-riding. Or, the bearings may freeze up from crud, rust, or wear.
Remove the pulley by unscrewing the hex-head bolt.
It doesn't necessarily have to be a store-bought pulley. You can take a pulley out an old broken derailleur, or mooch one from a friend.
Now insert the bolt and tighten it into place. Done!