The following video is for a front windshield on a earlier model mustang but is pretty much the same due to that it uses a rubber gasket. There also is another link that will help you some also.. You will want to get some closed cell foam tape to put on the window ledge first. I would also put your trim clips on before you put the window in.
MAKE SURE that you put the small metal "package trey" in first. If you don't the window will have to come back out to get it in. Center the window before you start. Once it's in there is no moving it. There was a couple of good install threads with pictures on the site but the links don't work anymore since the upgraded software. In the below video they show using a rope caulk inside the outside gasket edge. This is not needed. The foam tape seals it. I would put a small bead of caulking in the gasket channel the window goes in before I place it on the window. A little extra insurance against leaking. You will also be putting more caulking in between the gasket and the window once in place.
The foam tape is from McMaster-Carr. Part number 8694K77 - Blended EPDM with Adhesive-Back, 3/4" Wide, 1/4" Thick, 50'. You can use anything similar if you don't want to order this but make sure you use good quality foam. Remember this is creating a water tight seal. Just follow the info and take your time and things will go well. Not really that hard to do.
https://www.mcmaster.com/#catalog/124/3679/=1c2ueap
Mustang Windshield 1965-1968 Installation - Bing video
'71-'73 Back Glass Install (mustangandfords.com)