How to install a windshield that will not leak?

Silicone should never be used in a windshield installation. Butyl bedding compound is something you can put under the gasket and should take care of any leaks.

Urethane will work too but if you ever need to change/remove the windshield in the future you will need to cut it out greatly increasing the risk of breaking the windshield and the gasket will be destroyed.