I didn´t see the tut. is it nurbs or poly? if it´s a polygon model just open the duplicate option box and check instance. keep in mind to scale the duped object in a minus direction, which flips it over to the other side. means if you want to flip your obj over the X-axis you have to put (-1) in X-axis in the option box.

if it is nurbs you will have to select both corresponding vertices on each side (opposite of each other), then move them together or scale to increase/decrease volume.

hope that helped.