Flip the folded table up so that the red or black lock button faces up.
Flip the button and open the table fully. Flip the lock button back in the other direction so the table top remains flat.
Locate and press the circular metal button protruding from a hole along the length of the table leg.
Pull the leg out to the desired length.
Get under the table and look for a latch of some kind along the seams.
Rotate or press the latch button to release the lock.
Pull the table apart at the seam and insert the leaf.
Press the button or rotate the latch to lock the leaf in place.