Oh, forgot a couple of other hints/suggestions for you...
When installing the new window, prop the front edge of the top open a few inches to relax the fabric somewhat. Also, to prevent the fabric around the edge of the window from lifting back out from under the piping where it tucks in, run a thin bead of Permatex Flowable Windshield Sealant around the edge of the window before tucking the top material under. A new window should have double-sided tape around the edge that is designed to hold the fabric down, but the sealant will give you additional grip. Do the tucking in with the top completely up so that you have the tension to work against.