The belt goes around the underside of the turntable and stretches around the brass pulley on the drive motor. Just rotate the turntable till you can see the motor pulley and slip the belt off the motor pulley (if its still intact)and leave it around the turntable.
Next lift the turntable off its centre spigot (two hands either side of turntable and thumb pushing the spigot down)
Clean the brass motor pulley with meths.
Turn the turntable upside down and fit the new belt around the 8 inch diameter part and reverse the whole process. Stretch the belt around and over the brass motor pulley.
The new belt should be 23.6 inches circumference (or thereabouts)and about 3/16 inches wide.