How do you win the baby rocking minigame in Tomodachi Life?

Solution 1:

I usually hold it however I hold it normally so the screen is open to that first "click" and I just move up and down (closer to the floor then move up towards the ceiling) in small movements about the length of my stomach starting slow and getting faster until you find the right pace the baby likes.

Don't try to over think it too much.

Solution 2:

What has worked best for me is to rotate the DS so that it lays like a book. As I am right-handed, I grip the lower portion of the DS (the right side of the "book") with one hand and move the DS in an up-down (floor to ceiling) motion. I don't go very far - travelling maybe 6 inches vertically. The rate/rhythm I move at is faster than I would have expected, but the babies tend to be smiling with just a few small adjustments each time.