Kukkii-san Start by preparing your wig, as seen in my basic ponytail tutorial (steps 2 - 7...)
http://www.cosplay.com/photo/1856067/ ... then click on the thumbnail that says "newer."

In short:
- Pin the wig to a wig head that is your size. Make sure it's stretched real tight.
- Mark the spot where you want the ponytail to be.
- Section off the bangs / whatever hair you don't want to be pulled up in a ponytail.
- If necessary, thicken up the wig with extra wefts to hide the gaps between wefts.
- Sew a weft to the inside of the wig, all along the back / nape of the neck. Train the fiber (with heat) to point straight up.

You'll need to remove the wig from the head to add the superstructure, but before you do, please make sure it's thick enough and fits you well. You will not be able to alter the size of the wig once you've put it in a ponytail!