Feed any electrical wiring through the mounting block's center hole and test-fit it against the siding over the electrical box or vent pipe. The notches in the back should fit tightly against the vinyl siding's ridges. Remove the mounting block and place it face down on a workbench.
Run a bead of silicone caulk around the mounting block's top and side edges. Do not put caulk along the block's bottom edge. Place the mounting block back on the vinyl siding, feeding any electrical wires through the center hole. Press the mounting block firmly in place.
Make any necessary wiring connections between the house's electricity and the item you are mounting. Center the item over the mounting block. If you are mounting a light or doorbell, secure it to the electrical box with the supplied screws. If you are mounting a vent cover, line up the cover's screw holes with the mounting block's screw holes and drive a supplied screw through each one to secure the cover in place.
Apply a bead of silicone caulk along the seam where the item you just mounted meets the mounting block.