Squeeze enough silicone caulk into each hole to provide a moisture seal. Tap the lag screws in with a hammer to bite into the framing. Then finish driving the screws with a ratchet. Then push it down over the top of the ledger board. If you use 2 or more pieces of flashing, be sure to overlap the joints by several inches and use caulk to seal the joints.
Flashing comes in a couple of sizes.
Make sure the back's high enough to cover the gap between the siding and the ledger board and to ride up behind the siding about an inch.