Lift up slightly on the pullout leaf. The lifting action will disengage the leaf from the under-table slot. Some table designs require you to lift up on the table top to release the pullout leaf.
Pull out the leaf or leaves. If you need to expand the table just a bit, pull out the leaf on only one side of the table. For a larger group, extend both leaves.
Lock the leaves into place. Unless there is a manual locking system, you should just listen for a firm click to indicate the leaves are secure in their extended position. If the locking system is manual, consult the product instructions for specifics on how to secure the extended leaves.