Keep in mind that you just need to activate the four cardinal directions twice (up, down, left, right), but not necessarily by themselves, such that left-up will count for both up and left. So you don't need to do a full 720 in order to do his ultra (or a full 360 to SPD, for the matter) .
Your best bet is probably to go to training mode, enable input display, and then give it a try. That should show you where you are going wrong. A tip to get started would also be to neutral jump, do the motion while air-borne, and then hit a button when you land.