The mesh is held on to the vent by VHB tape (included). You can actually install the mesh with the vent still installed on the SS.
I decided to take my vents off to install as I was actually looking to see if I could countersink the vents so that they mounted directly to the undercarriage and be flush with the hood, but that's not really feasible. I don't think I could cut a nice straight line to be pretty enough when complete and I think the vent is thinner than the hood plastic itself, so it wouldn't fit flush anyways without a spacer.