How I did my wig, which is just past shoulder length, was take about 6 sharpies, cracked them all open and cut open the inkwells, and put them in some 70% rubbing alcohol. Shook it up a lot, and then dumped it into a bucket, and squeezed excess ink out of the inkwell.
Then I let my wig soak in it overnight, then dry the next day while laying flat. Then rinse until the water runs clear. I used 32oz of the rubbing alcohol and still needed to have used more since the wig didn't entirely get covered and I occasionally had to dunk it and flip it around in the bucket.
Hope that helps you some.