Dair autocancels from a full hop, so try using dair as soon as you start doing a full hop (starts at frame 4 after pressing the jump button), and you should autocancel upon landing. It's a pretty useful move for predicting the jumps and for shield pressuring (It's almost impossible to punish it on parry itself unless the opponent has a frame 3 option out of shield).
Landing the last hit of autocancel dair is a pretty underused but great combo starter, it can combo into uair easily. Landing dair has some uses, mainly to combo into up smash at KO% and possibly jab at low%.
It's a good move but it also has bad sides: high landing lag means it's punishable on shield if it doesn't shield poke and the opponent is aware of it (they keep shielding after they attempt to parry it), and its hits aren't very consistent, meaning that the opponents may fall out of it if you drift too far while using the move.