Here's how I did it

Step zero: Remove the rear seat so you can get serious about the install.
Step one: Get a friend to help. Be prepared to buy beer later. You are in the car, the friend is out.
Step two: Put dish soap around the perimeter of the window seal where it intersects the pinched, spot welded sheet metal ridge defining the window opening. Forget the rope.
Step three: The Assistant firmly pushes the lower rear sharp-cornered part of the seal down and back into the window frame until it is fully seated.
Step four: While the assistance holds that firmly in position, lift up on the lower front corner until you can finally slide it down onto the pinched ridge and then down into its respective corner. Be exceptionally careful to protect the rubber flap on the leading edge of the window seal as you guide it inside the car.
Step five: Working with the assistant who holds the outer seal surface as you proceed, work the front vertical edge, bottom edge and arching top edge properly onto the pinched sheet metal defining the window. Use your thumbs and fingers to do so. Ignore the small cuts of skin.
Step six: You end up at the top front corner which has yet to seat. Then slowly manipulate the rubber seal into place as you work from the inside of the car. The Assistant will apply vigorous outside pressure to ensure every hard-earned mm stays in place as you work this last section.

Regards
Will

1968 123GT