If you press [Triangle]/[Y] when falling/jumping, you will perform a plunging attack with your weapon. Should this connect with a monster (ideally the monster’s back - glancing blows may not suffice) you might mount the monster. An easy way to do this is to fight a monster near a ledge - it need not be very high at all - and dodge off the ledge towards the monster, which should automatically cause you to jump and clear the ledge, then simply press [Triangle]/[Y] in mid-air and hope for the best.