I pay to them done, because once you break one windshield, you haven't saved any money. As a buddy used to say, "You're chasing a nickel with a dollar."
I had a new windshield installed last year. My gawd, is there anything nicer than a new windshield? Anyway, the glass guys put the gasket back on the cab, set the new glass against it, and using glass cleaner as the lubricant, set the base of the windshield into the seal, and the worked up both sides at the same time, and then they headed towards the center of the top, from each corner.
Never do you want to install a windshield from a corner first. You'll break them every time. Always start at the bottom, whether using the rope method, or the seal-goes-in-first method, and then work evenly around it.
Speaking of chasing a nickel with a dollar, I decided to save the $50 or whatever, because I thought my seal was good. Well, now it doesn't fit against the cab good at all, and guess how bad it leaks. Thanks for the tip on the rockauto gaskets. I need to get mine replaced.
Oh, regarding the suggestion to use silicone. Don't. There's something about silicone and auto body paint. Plus, if you ever have to replace that, you'll be screwed. There are several butyl based, black glass sealants made for this, and they are readily available.