Put your extra leaf near the table.
Tug gently on one or both ends of the table until it separates at the center. If you can't get the table top to separate by pulling just one end, you may need to enlist some help.
Locate the row of pegs on one half of the table and holes for pegs on the other.
Find the pegs and holes on either side of the leaf.
Set the leaf into the space that has opened up, so that the pegs in the leaf face the holes in the table.
Plug the leaf pegs into the table holes, and set the table's pegs into the leaf's holes.
Shove the ends of the table toward the middle to close the spaces between the elements of leaf and table.