If I understand correctly, you want to know how to cut the cloth before you build your deck.

The cloth that is hanging over will absorb resin that runs down the sides, and depending on how much that occurs will determine how hard it is to cut through. Once flipped over, I can usually cut the cloth with a sharp utility knife - running it along the edge of the sheer line. To clean up the edge further, I use a simple surform scraper (wear safety glasses) and then even plane the edge. Be very careful - this edge will be very sharp once cut with utility knife - don't slip and wear gloves.