Turn into the Morph Ball, and slide off a platform or any other surface that allows Samus to stand in the air as Morph Ball. While in mid-air, press UP, and Samus will now be standing on her feet without any ground below her (She will be doing this still in mid-air), then quicly press ''A'' to perform an jump from the standing position you were placed. This trick will only work as long as Samus's sprite is stationary, this means that if you hit the D-Pad from you standing position, you will not be able to Morph Ball Double Jump.