I think he means SB as in shield breaker.
OOO...There are several methods to doing it. If one doesn't work for you, try another. Let me list the ones I know of.
Joystick tap down, jump, neutral b.
Cstick back or down, jump, neutral b.
Pivot back to foward, jump, neutral b.
Joystick back circle to half way between up and forward, neutral b.
I recommend either pivot, y, b or tap down, y, b. If you play a spacie, this should be fairly easy for you. I have trouble not hitting the A button when doing this, so I shy away from this method.
I personally find that the cstick methods are the most difficult, so I use the joystick back circle method. M2K, because of the way he holds the controller, taps down then uses x, b (He bends his right index finger over x to do it.)
Pivot is much easier than tapping down because if you miss with tapping down you are basically dead, and every single marth should be well versed in pivot grabbing so the pivot should be no problem. It's up to you whether you want to jump with up, x, or y, but pick a method and perfect it. If you need a video tutorial on any method I can do it for you, but I've personally done every single method I've listed so I know for a fact that they work.
Also, you can fsmash tipper bowser on a platform of DL in case you didn't know. This game is funky like that. (I've known this for a while but finally confirmed it worked on bowser only last night.)