Release any locking mechanisms on the underside of the table by rotating the levers.
Open the center seam by pulling one half of the tabletop away from the other half. You may need the help of a another person pulling from the opposite side. When the center seam is open, you should see holes drilled in one side of the seam and wooden or metal pegs protruding from the other side. Pull until the seam is parted far enough to receive the filler leaf or leaves you want to install.
If the table has a single "butterfly" leaf stored within the table base, lift and open the leaf flat. Push the table closed so there are no gaps. Otherwise, proceed to Step 3 to install separately stored leaves.
Examine a leaf, noting the pegs on one side and holes drilled on the other side. These will line up with the pegs and holes on the seam edges of the table.
Place the leaf, finished side up, between the opened halves of the tabletop so that the pegs on the leaf fit into the holes on the side of the tabletop center seam. If you add an additional leaf, be sure the pegs on the added leaf fit snugly into the holes of the leaf previously placed.
Push the tabletop from either end to close until there are no gaps.
Reengage any locking mechanisms on the underside of the table and the leaves by rotating the levers.