Install a ledger board, a 2-by-4 that will serve as a ledge or nailing support for the wall end of the lookout. Place this on the wall to match the level of the end of the rafter end beyond the wall. Use a tape measure and level to determine this placement, which will vary with the length of rafter overhang and its slope. Put a level at the bottom of the rafter rail to the wall and mark a spot for the bottom of the ledger. Mark this at each end of the wall. Snap a chalk line to mark the bottom of the ledger board.
Nail the ledger board with a hammer and framing nails to the wall studs, through the plywood or other sheathing on the wall. Use framing nails long enough to penetrate the studs through the sheathing; that length will vary with the type of wall sheathing. Nail one end of the ledger, then level it along the chalk line and nail it at the other end. Go back and add two nails at every stud.
Use a tape measure to determine the length between the face of the ledger board and the end of the rafter. Cut 2-by-4s with a circular saw to that measurement for lookouts, square on both ends. Install one lookout at a time. Nail the end of the lookout to the rafter tail through the side with one nail, then toenail the other end to the ledger board; toenails are driven at an angle through the lookout into the ledger. Add a second nail on the rafter end once the lookout is level and secure.