Unlock the dining room table, if applicable, and pull the dining room chairs away from the table. Then, stand at the far end of the dining room table with your helper at the opposite end.
Pull your end toward you while your helper pulls toward them until the dining room table splits to reveal a gap large enough to insert a dining room table leaf.
Slide the dining room table leaf into the gap, securing it in place according to the manufacturer's instructions (many dining room tables use a corresponding hole and peg system for securing the leaf).
Push the dining room table back together from each end until the dining room table leaf is fully integrated. Reposition the dining room table and put the chairs back in place.
Find a small table that is the same width and height as your dining room table (a folding table works well). If you don't already own a table that will work, you can rent, borrow or purchase one.
Move the chairs away and then move the dining room table to make room at one end of the dining room for the extra table. Push the extra table firmly against the exposed end of the dining room table.
Spread a long tablecloth over both tables so it looks like one long dining room table and put the chairs back.