I have done a little braiding so I will try to help if I can. First if you are planning on doing flat braiding, you will only be able to braid as long as the hair is, you can't add to the braid. If you are planning on doing a round braid than you can make the braid as long as you want. With horse hair all round braids come out square, they are usually done as a 4 braid , 8 braid , or 12 braid. To braid, you will have to make what are called PULLS. To make a pull take a number of hairs and hold the thick end with your fingers, then take another set of hairs and put the thin ends next to them, keeping them seperated. Tie the end together having the long ends hang over one of your fingers. Then take the tied end and twist it. Keep twisting, keeping the two groups seperated. When you get to tghe end tie it off. Putting the pulls together thick end to thin will give you a more consistant pull. Different colored hair have different thicknesses. For black hair I like to use a total of 14 hairs per pull, 16 hairs for brown, and 18 hairs for white.
Braiding is done just like with leather. 4 braid is under one over one, 8 braid is over 2 under 2, 12 braid is over 3 under 3. The best way to do it is to use two pulls as a core. start braiding. After going at least one inch take the third pulls down on each side and pull them down to where the core is, these will become the core, them take the core strings and pull them up to take there place. continue braiding at least one inch. when these new core strings get short, cut them off and add two new pulls as cor strings. After braiding for an inch you can use these to extend the braid. For 12 braid always use 3rd string from top, 8 braid always use 2nd string from top, I have never tried to extend a four braid, but it would probably be the top string.
I'm sorry for the rough explanation. This is harder to explain without pictures than I thought. If you are really interested there is a book called braiding with horsehair written by diane gadway and richard schneider. It is only about 25 pages, but is very informative and is only 10 or 15 dollars. you can check the web, there are a few people that sell it. My descriptive abilites are not very good. If you are interested I could try to post some progress pictures.
The pictures I have attached are all multiple 12 braids sewn together.