It's very easy. Just cut a patch out of 6 or 8 oz. cloth. Saturate it with 30 min. epoxy and stick it place from the inside of the cowl. Make sure you rough the surface inside with 100 grit sand paper beforehand. When dry re-coat it with epoxy to fill in any holes and then paint it.....I usually paint mine black.
If you leave some slack in the patch you can make it concave by pushing with your finger from the outside just before the epoxy sets up making the deflector I mentioned which slopes toward the engine side. When the propwash hits it is deflected or channeled toward the engine side of the cowl.
PilotFrog