It's really easy.
Go into training mode and move the computer to the edge. Then, away from the computer, do a dash.
From that, do a shield.
While still holding shield, press A, but make sure you press A AFTER the shield.
Then practice doing that faster. If Dedede does a tumbling motion, you're doing it wrong. Keep practicing that until you can do it consistently.
Then once you get that bring the computer in to the mix. Do the same thing, but then press down afterwards. So it's Dash -> Shield ->A ->Down. If you do it right, going from one end of the stage to the other, you should get them to around 40% damage.