Which is the correct side? Generally, the mane laying on the right or off side is considered correct. If you pleasure ride or show in very low level shows, which side the mane lies on may not matter. Folklore suggests that a horse's 'handedness' is indicated by which way the mane lies at the withers.
Comb out the mane so it is neat, tangle free and all lying on one side of the horse's neck. Dampen it slightly (a sponge or spray bottle works well). Divide into sections about 1 to 2 inches wide. Braid each section and secure with strong thread or elastics. (You might want to pick up a package of small braiding elastics at your local tack shop.) Don't braid the tuft of mane at the withers. It doesn't matter if the braids look nice. You just want the weight of the braids to encourage the mane to lay on one side.
After a week's time pull the braids out, comb out the mane and re-braid if necessary. Some thin manes will lay flat after a week—some can take a few weeks.
This is the only time you will want to leave braids in your horse's mane. If braids are left in for a long time the hair will break and tangle, leaving your horse with a frizzy uneven mane. And if you've ever worn braids yourself, you know how uncomfortable they are after a time.