Remove the broken slat by using your screwdriver to unscrew the screws that are holding it to the brackets on the top and bottom rails. (Remove and replace any broken brackets.)
Clean the top and bottom rails and their brackets of any dirt or glue with a damp cloth. Hang the replacement slat onto the brackets of the rails.
Screw the new slat into the brackets of the top and bottom rail using screws and your screwdriver. Apply adhesive to the slat and the brackets and clamp the slat onto the top and bottom rails for at least an hour. Let the new slat dry for 24 hours.