first you have got to take the door card off and clean out all the glass.
there is a track that your glass would have been fixed to that needs to be unscrewed from the regulater. your second hand new bit of glass should come with it on ( if not you can line it up to correct possition, soap it up and bang it on, rubber mallet prefered)
put it into the frame front end down lower, pull it up square, slide the glass down onto the regulater and put screws back in.... i know this as this was my job for twenty odd years, i hope i have the correct door in mind though