To remove: Pinch the tabs on each side of the belly bar where it attaches to the seat and pull in to release. Do this one side at a time. Once the side tabs are free, push the bottom tab through the seat's base, then pull the belly bar up and out.
To attach: Place the center piece through the seat (the outermost slot). Then put the right and left tabs through the holes on the seat. You will hear it snap into place once secure.