YAY!! I love myself some tissue balls - but TULLE Pom Poms complete the TuTu themed baby shower idea :)
Twine / Rope
Depending on the size of the pom pom you want - cut your circle into your cardboard. Put a piece of twine/ribbon around the inner circle (make sure to be able to see the twine/ribbon).
Then start to wrap tulle around the circle until you have your "thickness" you are looking for. Make sure to leave your twine hanging out - so you don't cut it. Then cut the tulle around the circle.
Then tie the twine into a knot - pull the cardboard off - and POM POMS!