I've just finished replacing three out of four this season; one more to go. I replaced them with plexi, but I figure in a few years I'm going to splurge on real opening portlights anyway, so the cost for lexan isn't worth it.
I removed the old one first to use as a pattern. It's just held in with caulk, which could be removed with unfortunate ease and the window pushed out. Getting the last of it out of the cabin top was a pain and getting the surface clean was a pain; I found copious amounts of acetone worked fairly well.
I then masked off the inside and outside of the cabin top, as well as the inside and outside of the window itself, with painters tape. I put a bead of cauk (The yard recommended SikaFlex which I used) around the inside frame, pressed the window in firmly, and then ran a bead of caulk around both the inside and outside frames. The cabin top around the two forward windows is slightly curved; I personally didn't find a problem getting the window to stay put and didn't need to resort to bracing it against a stanchion to bend the window.
Someone on this board recommended using a finger dipped in soapy water to smooth the caulk while it was wet; I did that, and it worked nicely. Then came the waiting game of peeling the tape off to leave a fine line. Too soon or too late makes a mess.
It doesn't look as professional as I'd like, and the SikaFlex seems to attract dirt, but I can see out the windows clearly and my water leak problems are gone, so I'm happy.
