I have a long wig like this. Here's how to do it, the poor person way.
Fill the sink up with waer and wash the wig in it. You don't need shampoo, just condition it. Or use detangling spray. With some conditioner/water in it still, hang it and brush it from bottom-up wih a normal hairbrush to get out normal tamgles. Comb it with a wet comb that's laced with conditioner. DON'T LET THE WIG DRY WHEN IT'S BEING BRUSHED. Wrap it in a towel to dry. It's fine to have some conditioner in a wig.
Whe you aren't usuing it, put ponytails at the bottom where the hair ends. If the bottoms can't move, the wig can't tangle.
Something else that I've heard is that once you have it styled is that you should hairspray it into place. This might be common sense or sound absolutly stupid; I'm not sure. That's just what I heard but it might be worth looking into ^^'
WHATEVER YOU DO! dont use a plastic comb (Might work if you constantly wet the plastic comb though) , or a brush! The color will fade and a lot of hair will be lost! But theres a special Wig comb you can buy! You will need one, if you dont want the wig to be ruined on the con, so you have to constantly comb it xD! Mine got ruined because I forgot my wig comb at home lol had to throw away the wig when I got home, it was such a mess xD 120 cm hair is a pain to take care of, dont know how Miku does it LOL!
wowz, don't kill the wig!!! D: I've lost 4 already. ||D Wells, if it gets tangled then use some sort of conditioner thingy or something like that to make the wig smooth and softish, and then start from the bottom and then comb it out. :3 this rescued 2 of my wigs from the dead! 4 of them were so bad that I couldn't save them And then yeah... oh and sally's sells really good synthetic conditioner stuff! And if you're gonna wear your wig around a lot, try to bring a comb and some conditioner stuff with you, so that you can keep it nice. :3 and try not to let the wig rub up against your body too much, cuz friction gets it frizzy and tangled.
My girlfriend (an avid cosplayer) says that you can soak the wig in a warm water + fabric softener mix, that seems to help loosen the tangles. Use a wet comb (dip it in water, then brush, then dip again, etc) to straighten it.
Also, you should braid the hair when the wig isn't being used. That will help a lot in terms of keeping it from tangling.
Most wigs nowadays are thermally-resistant, so you can use a hair straightener to straighten them out if they get frizzy. Don't do this with wavy or curly wigs that come pre-styled.