On the 300E, the fan blades and fan shroud will have to be removed. It can be done w/o doing these steps, but it's almost impossible to get to the tensioner bolt w/o at least moving the shroud out of the way. At any rate, next step is to slack off the big bolt in the center of the belt tensioner. Back off the the tensioner adjusting bolt (sticks up near the power steering pump resevoir) take off the old belt and reverse the process to reinstall. If you don't have a routing diagram, take notice of how the belt is routed around all of the pulleys. Good luck.
1987 BMW 325