Continue raising the mainsail with the halyard as you insert one slug after another into the groove.
Note that this sail already has its battens in place. A batten is a long, thin, flexible strip of wood or fiberglass that helps the sail keep a good shape. They are positioned in pockets sewn into the sail in a generally horizontal direction. In this photo, you can see a batten near the top of the blue section of the mainsail over the sailor’s head.
If the battens were removed from the sail, you would insert them back into their pockets either before beginning to rig the boat or now, as you raise the mainsail in stages.