Pull the extender leaf from beneath the table. If you've stored it separately from the table, make it handy.
Pull either end of the table so the two halves begin to separate. An extender dining table is designed to separate at or near its center so the extender leaves can be inserted. Once separated, you should see interior pegs on one side of the table top and holes into which to insert pegs on the other side.
Place the leaf into the gap, positioning it so its pegs and holes align with the holes and pegs of the main table. Even if you've managed to pull the table apart on your own, this step requires another set of hands.
Insert the pegs of one side of the leaf into the table's peg holes.
Insert the pegs of the table into the leaf's peg holes.
Push the ends of the table toward the center until no gaps remain.