To perform this trick, simply perform a dashing Jump (jump while dashing), then release all buttons and wait for Samus to fall near the ground. Right when you see she is about to make contact with the ground, hold Jump, and Morph (by pressing Down twice). You can actually press Down the first time at any time during your jump - it's only the second time that must happen right when you hit the ground. You should begin rolling as the Morph Ball at the same speed you were jumping (hence the name).
Examples of the Mockball include getting Super Missiles early.