How do you tighten a fan belt on a 1971 volkswagen beetle?

Fan belt tension is set with shims. Look at your generator. There's a nut (21mm) on the shaft, and on the back half of the pulley (the one away from you) there are two slots. You stick a screwdriver in one of the slots to hold the pulley, and remove the nut with a 21mm wrench or socket. Slip the front half of the pulley off, and you're ready to start adjusting. There are, hopefully, two or three shims between the two halves of the pulley, and two or three more under the little cone that's between the nut and the pulley. To make the belt tighter, remove a shim from between the pulley halves and put it under the cone. To make it looser, put one of the shims under the cone between the pulley halves. If you don't have any shims between the pulley halves, you must replace the belt. To reassemble the car, have the shims you're going to use on the generator shaft. Loosely loop the belt over the shaft, then put the front pulley half back on, followed by the spare shims, the cone and the nut. Start tightening the nut without holding the pulley stationary. As you tighten, the belt will move into position. You want the belt to have about 1/2" of play halfway between the pulleys.