According to the tutorials I found, you should use 1 cup of surgical spirit mixed with 8 to 10 squirts of the ink - there a dropper in the lid of the ink which you can use to suck up ink. I’ve been warned against using colours containing white ink, so I used a cerulean for the blue tail, and a mix of red and pearlescent pink for the other tail (8: 2 ratio of red to pink ink).

I mixed the ink in old jar and carefully dipped the ponytails in. Later I transferred the mixture into old plastic and styrofoam trays (the kind you get frozen meat in - clean it first!)

I left the wig to dye overnight, then carefully rinsed out each tail in the sink - it’s made of ceramic so is much easier to clean than the plastic bath. While I rinsed the blue tail, I used a hair clip and a plastic bag to keep the red tail from dripping everywhere. I rinsed out the ponytails with a mixture of shampoo and fabric softener. 

Once they tails were dry I combed them with a wig comb. Be warned that if you use normal detangling spray it may lift some of the colour from the dyed wig, and it also gave my blue tail a strange texture. In general wigs dyed with this method have a slightly rough texture, but if you rinse them thoroughly they should be fine.