OK-Here is how I have learned to do it, and thanks to all of you who have replied.
Step 1--if you don't already own one, go to an auto glass shop that will sell you a tool called a "bone".
Step 2--grate a bar of soap into shavings and make a slurry of soapy water.
Step 3--cut the gasket to size.
Step 4--use a paint brush to lubricate the interior slot of the gasket with some slurry.
Step 5--insert the pane in the interior slot of the gasket
Step 6--cut a length of 1/4 inch line, or whatever dimension fits snugly in the exterior slot of the gasket, long enough so that you have ends hanging out long enough to get a firm grip on-- you can use constrictor knots around the middles of a pair of 3"-4" dowels to make the pulling extra easy.
Step 7--slurry the exterior slot, and have one person put pressure on the top of the unit from the outside, making sure that the solid side of the gasket is outside, the locking part on the interior, and that the gasket is unlocked. The person on the inside (the strongest is best) works the gasket onto the hull edge, starts slowly pulling out the rope while the unit is being forced down, helping to spread the gasket onto the hull edge with the thin end of the bone. Work around the port hole in both directions.
Step 8--when the gasket is on all around, slurry the locking surface and, using the wedge end of the bone, tuck it into its slot in a series of jabs--it won't work if you try to just run it around.
Step 9--wash and dry the portlight. That's it. I have done one (5 to go) using this method so I know that it works, but it does take strong fingers.
Thanks again to those who have shared advice.