Cut the end off a cotton swab in order to transform it into an RTV applicator.

Alternatively you can try removing all of the cotton from the swab, but you certainly do not want any cotton fibers to remain on the swab.

Dip the swab into the RTV silicone, and use it to apply the RTV silicone to the underside of the subwoofer tear.

You do not need a huge amount of RTV silicone on either side of the rubber surround — just enough to form a very thin layer.