You can just plait it all on the side you want it to be one. Just break it into medium size bits and plait it and leave it for a week or so and it will stay over there. My wb has a really think mane that went the length of his neck on both sides and it was sooo wirey. I didnt want to pull it because I don't like doing that so I conditioned it like mad and plaited it up for a week and now its lovely and stays on the one side.
The most effective method (at least for my horse) was to get the mane clean and wet, and then braid it, but only go about half way down the length of the mane. So, instead of braiding it all the way to the tip of the hair, don't go any further than half way. It laid a lot flatter that way. I'd keep the braids around the width of a finger. I'd leave it in for about two weeks and it would stay exactly where it was supposed to for months.
Braid it. Do about a dozen braids (the way you would do a braid in someone's hair, not showing braids) to the side you want and leave them in for a least a week. Repeat it or leave them in longer if needed.