rips between your fingers and the base of your thumb, ans shown in the picture.
Slowly, stretch the strip. It will begin to "neck out" at one location on the strip - the "neck" will then propagate up the length of the strip as you pull on it. See before and after picture.
Repeat the process with all your strips.
This might sound all very easy, but it takes practice. The strips frequently break before your done pulling them, so if it doesn't work at first, be patient and keep trying.
Cut the stretched strips into sections slightly longer than twice the length of the Styrofoam cups, leaving some extra length for error.
NOTE: Don't expose the stretched strips to direct sunlight, as it will per-shrink them. However, if the sun is out while you are working on this step, try laying laying one of the strips in the sun - watch it shrink!