Does anyone has instructions on how to replace the rail weatherstrip on a 1974 Cutlass Supreme

 

Strip out the old. Clean the channel well removing all traces of glue and rubber. Then install the new seal. Use 3M weatherstrip glue sparingly and only along the top of the seal. Apply glue to the seal. If this isn't a repaint then no need to remove the stainless rail. If it is a repaint then the rail will need black strip calk applied between the roof and the rail. Use a popsicle stick or other non-metal tool to help tuck the seal into the channel. Don't use a metal tool.
You might need to adjust the window. But wait until the new seal settles. Close the door first then roll the window up to see how well it contacts the new seal. Leave it closed for a week to "break in" the seal. Then try opening/closing the door with the window in the up position. Make small window adjustments as needed. Lube up the window regulator while the panel is off.

You don't have a movable rear quarter as seen in the 68-72s. For those models that have this, the metal guide riveted into the channel above the quarter window forward seal with the rubberized coating on it can be restored using many dips of plasti dip. Strip off the old plastic material first. I mention this for the benefit of those who will look at the thread with this guide.