Ahem.
Brawl has a 10-frame buffer window and moves at 60 fps. This means you have 1/6th of a second before your animation finishes to input control stick to the side, let it return to neutral, then when it's returned (or while it's returning), C-Stick up twice.
Yes, you have to let the control stick return to neutral before the FIRST C-stick. The idea here is you want to input a dash attack, then an upsmash, during 1/6th of a second. If you hit control stick and c-stick at the same time, the game reads running upsmash. You have to input the first c-stick while the control stick is returning (or has already returned) to a neutral position. The second c-stick inputs the upsmash.
If you're trying to learn BDACUS, I recommend first analyzing the timing for the move you want to BDACUS after. For those of you still learning, I recommend Dthrow.
To start, try buffering a dash attack using these controls with Dthrow. Perform your Dthrow, then, roughly around when Falco shoots his very last laser downwards, input control stick to the side, return to neutral, c-stick up. If you UpSmash, you inputted the Dash too soon, or you didn't allow the control stick to return to neutral. If you sliding UpSmash, you inputted Attack too late. Once you can consistently do an instant Dash Attack out of Dthrow, move on to BDACUS. All you're doing is adding a C-stick up after the first two motions.
While practicing BDACUS, if you...
UpSmash - You are performing the Dash, and probably the first C-stick, too soon or too close together.
Sliding UpSmash, but not very far - You have the motions perfectly, but are not inputting them fast enough.
Dash Attack - You have the motions properly but are not inputting them NEARLY fast enough. Specifically, the second C-Stick is coming out too late to cancel the Dash Attack.
Laser - Something's wrong with you.
*vanishes again for another few months*