You wanna grow a tail? What you put in your horse will out, and not only in manure ;). Skin, shiny coat, mane and tail growth and quality, hoof quality...all linked together in dietary needs. Omega Horseshine is a good supplement that covers many needs, and the flax in it has the same action as psyllium to get rid of buildup in the gut. If it's cold
make sure your horse is getting some extra calories from fat to stay warm and grow hair.
Put tail in bucket of water and wash, include a gentle soap if you want. Dry tail. Add a leave-in conditioner spray or apply a natural oil (coconut or sesame oil.) Separate your 3 braiding strands and brush them individually (start from the bottom, and work in small bunches). Trim 2" off of each braiding strand. Braid below tailbone, leave the last 8" of tail unbraided. Add scrunchy. Fold tail, making scrunchy touch just below the end of the tailbone. If that's not going to be short enough, you can fold the braid twice. Put that neatly into a stocking (stocking should be long enough to gently cover everything, up to the dock of the tail.) GENTLY
(not tightly) cover the upper 2/3 of the sock sock with vetwrap. (If you don't do this, the horse can rub the tailbone and break hairs. Assume your horse will try to rub.) Add one strip of duct tape where the vetwrap ends to make sure it doesn't unravel. Voila! You can leave it like that for 2 days...possibly longer, but I wouldn't. You'll want to let it get some air and de-compress it with a light brushing. If there is excess oil or substance left, remove with towel. Remember that the tail you eventually get could very well need extra care and attention - if only to save it from the horse it's attached to.
Conditioning products that I like (if not natural oils): http://www.horse.com/item/eqyss-prem...spray/E013642/ http://www.horse.com/item/mane-n-tai...ler/SLT054589/
If you wrap, and if flies are an issue, there are tail 'addons' that look just like tail and act like a whisk. That sounds really tacky, even hearing myself saying it, but hey - don't knock it if it works. Or just keep a fly sheet on your horse for a while.
Careful about putting too much product on the tail - it can cause buildup at the skin level, and possibly cause a dandruff-like situation.
Whatever you do, I do not advise simply braiding the tail and doing nothing else with it. Everybody I know who has tried to grow out a tail simply by keeping it in a braid, ended up with hardly any tail at all. And these were horses kept separate from one another - no herd play or chewing of tails.