The packet with the screws and instructions is commonly tossed with the packing material.
The screws are #8 x 1" Flat Head sheet metal screws.
You drill through the deck, sleeve and backrest leg with a 5/32" bit. Pull the backrest out and enlarge the hole in the leg to 3/16". Re-insert the legs and thread in the screw. The tip of the screw would sit in the 3/16" hole loosely to allow for the seat to move a bit, but it keeps the seat captive in the crossbars. You can also use quick pins.