I don't remember exactly on my Pinto, but on most cars with a v belt you loosen the alternator. Normally  there is one bolt that passes through a slot on the mount and when you loosen it you can move the alternator toward the block which will allow the belt to be removed. At that point you should be able to remove and replace the p/s belt  if the belt is run through a double pulley on alternator. If not, you may need to loosen a bolt on the p/s mount also. Then just place new belt into place, pry the alternator outward and tighten the belt and then tighten the bolt to hold it in place.  Don't over tighten the belt. You should be able to push on it and move it about a half inch.  Having said all of this I will also say that I have come across a few engines that requires the removal of another belt first in order to remove and replace the belt.  I'm not good at explaining things so I hope I am not just confusing you.