If you choose to cut from the outside you might have a problem when the windshield has been placed too close to the pinch-weld. If there is less than 1/8” of urethane then the knife will not have room to drag properly. This causes the glass to break and make a mess. Cutting the windshield out from the inside of the vehicle is the other option. You can use an extended handle razor knife and cut with a repeated dragging motion. Many installers also use power cutters which are faster but do more damage to the metal pinch-weld.
The best thing for the new urethane to adhere to is the old urethane. It must be clean and free from dirt, oil or other contaminants. One problem installers have is wind blowing dust onto the pinch-weld before the urethane adhesive has been applied. The job can be done without an electric gun but it is much more difficult to get a consistent bead, making leaks likely.
Some vehicles have mounting block for the bottom of the windshield to rest on, others do not. Be careful not to touch the frit band because the oils and dirt from your skin can contaminate the activated glass and reduce adhesion to the urethane. Some installers tape the windshield once it is in place. This holds it until the urethane dries.
Spray the inside and the outside of the gasket with glass cleaning to help lubricate it.
