Unscrew the casing from the plug and then solder the red and black wires to the mono plug. The red wire should go to the small tab (audio). The black wire should go the larger tab (ground).
Pull the shoe lace as close to the plug as possible and then slide the cover back up towards the plug.
Make sure none of the connections are touching. Pull the cover up right to the base of the plug where the shoelace meets the wire connections. Fill the cover with hot glue and screw it on as quickly as possible. You may need to use your pliers to make the last few turns.