Take the mount out of the car.
Cover one side of the mount with tape. Duct tape, aluminum duct tape, lacquer masking, doesn't matter.
Set the mount on a nice, flat, stable surface.
Mix up the urethane, pour it into the mount (except the bolt holes, obviously).
Let harden for the specified amount of time (some are 48-72 hours).
Remove tape, put the mounts back in.
Basically, unless you buy a backup set of mounts, you'll be without the car for about 2 days.