SDM, right? Well, you need to be EXTREMELY close to your opponent, and then you need to do the motion very quickly and accurately. Don't spazz on the pad/stick, as that never gets you anywhere.
As for the regular dm, just do qcb, qcf, f + AB, or simply qcb, hcf + AB. Also, the SDM requires that you press the B button only, if you didn't know that already.
HIBIKI RULES!