This is a really important throw and an excellent defence to a punch
When the punch comes in, instead of blocking with your left hand and stepping through with your right, cross-block with your right hand and step behind the attacker with your left foot, remembering to cover your face with your left hand.
Then use your left hand to push the attacker’s head backwards, while you sweep his legs from under him with your left leg (swap right and left in the instructions if he attacks you with a left punch).
Its easy to get it wrong so….
Remember to step behind
Remember its cross-blocking
Remember to step deeply
Remember to take your attackers balance by taking them back wards.
The really flashy can do tani-otoshi as a walk, executing the throw mid-stride then carrying on in their original direction.