When I did mine, I applied the tape to the window frame, then carefully pressed the window into it's hole in the fiberglass wall. It stuck in place while I went inside and screwed the clamp ring to the interior of the window frame. Then I methodically tightened the clamp ring, going from one screw to the one on the direct opposite side of the frame to equalize the pressure on the ring as I tightened all of the screws a little at a time until the Butyl Tape would squeeze out of the edge. This expansion of the butyl tape under pressure will keep happening for days. I'd trim the excess after a couple of days and check to see if I could tighten the clamp ring screws more...
Frederick - The Scaleman
1978 Fiber Stream 16 named "Eggstasy" & 1971 Compact Jr. named "Boomerang"