Are you doing the install or are they?
Do you have the new rubber seals and have you cut them yet?
I have found that I have the best luck when I have the cut seals and then make a template out of 1/4 luan. I fit all that into the window opening and tweak the template as needed.
I also recommend using O rings glue to re connect the cut rubber seals.
JMHO.
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Torchie