Normal proceedure is to put 3M window goop in the glass channel of your nice clean seal and install it on the glass. Then, wrap a length of poly cord (like, 1/8" or smaller braided, not woven) around the seal and window (down inside the cavity that clamps onto the seam in the body)... have the ends overlap a lot and have the overlap at the bottom.

Place the window in the hole such that the bottom portion of the seal is "started" and ensure the cord is inside the rig! Lube the seal with your favorite stuff, Ruglyde, armor-all, glyceryn, soap all work, all have proponents, all have positives and all have drawbacks.

I use a few pieces of packing tape on the exterior/top to hold the window while I climb in, I seldom have helpers.

Once in, hold one end of your cord and pull on the other. Periodically go outside to push on the window, toward the seal you just pulled out of the way at first and later (near the end) just push in... hard!

You will work the inner lip of the seal out and over the seam of the hole in the body as you go by pulling on the cord (hold the other end!). Alternate sides you pull/extract on.

Take care at corners (push extra after doing a corner) and be aware that you can cut the seal by injudicious/swift yanking on the cord.

Usually, the exterior glue-schmuckum gets injected into the seal after it is in place by using a nozzle and inserting it up under the seal from the exterior.

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Mike!