Kukkii-san Get yourself a pair of styrofoam balls (hollow or solid), or whatever shape the style requires - referred to as 'foamcore' or 'odango' from here on.

Carve a hole into the bottom of the foamcore. It must be wide and deep enough to accomodate both the sealed ends of the extensions that you'll be wrapping around the foamcore, AND the ponytail stub on your wig, because you will stick the finished (wrapped) odango onto that stub. (When in doubt, always make the hole a little larger than you think.)