First, soak the Vinyl Insert in a tub of warm (but not hot) water, with just a drop of liquid soap, for about 10-15 minutes. This will make the Vinyl Insert more pliable and easier to work into the threshold grooves. Once the Insert is warmed up, it should be pliable enough to slide into the threshold groove with no problem. If the insert is just being stubborn, a plastic putty knife or plastic spatula can help guide the insert into the grooves for a snug fit.