personally i would be cutting them holes now before fitting the frames, then when the windows go in u can seal all around the inside and out to make sure no water will get out from any where
thats just my opinion, u might have a better plan 
It might also make it easier for u to straighten the liner out properly as it has a proper grip with the frames keeping it in place


It might also make it easier for u to straighten the liner out properly as it has a proper grip with the frames keeping it in place