I braid with yarn rather than elastics because the yarn stays in better and creates prettier braids by helping to keep the braids as close to the neck as possible rather than having them stick out sideways.
The number of braids that you should do depends on your personal preference and on your horse's particular mane. Thicker manes usually require more braids to keep them a manageable size, while thin manes can require fewer braids in order to give more bulk and roundness to each individual braid. If your horse's mane is extremely short, you should take smaller sections of hair to keep hair from popping out mid-braid (the further the hair has to move sideways to get into the braid, the shorter that section of hair becomes compared to the middle section).
Before starting, get your supplies ready. You'll need yarn that matches your horse's mane, a spray bottle of water (or Quic Braid if you prefer), a pull-through/rug hook (available at tack shops and at craft stores), scissors, elastics (optional) and a seam ripper for removing the braids afterward.
To measure and cut the yarn, grab the end of a roll in one hand and start wrapping it from that hand around your elbow (on the same arm) and back up to your hand. Keep wrapping until you have about 12 to 15 wraps, depending on the number of braids you plan on doing. Find where you've grabbed the end of the yarn and cut through all of the wraps at that one point. This should leave you with 12 to 15 pieces of yarn of the same length, about double the length of your lower arm. Hang these in an easily accessible spot, either somewhere around your belt or on your horse's halter.
I recommend starting behind your horse's ears so that the higher braids end up more evenly spaced. If you reach the end and have to do one wide or several narrow braids, the difference in size will be less obvious at the withers where your hands and saddle pad will hide the braids anyway.
After spraying the first few inches of mane a few times, grab a section of it (I prefer to simply use the same hand motion every time to get the same amount of hair rather than using a comb) and braid it straight down. Straightness is important at all times because crookedness in the braiding process will result in a crooked braid. You can make the first couple of cross-overs fairly loose if you would like to give your horse some room to stretch and then braid tightly the rest of the way down.
About 1/3 of the way from the bottom, grab one piece of yarn while pinching the braid tight with your other hand and fold the yarn in half. Lay it behind or in front of your braid so that the middle of the yarn is against your braid, and continue to braid the rest of the way down, incorporating the two ends of yarn into two sections of hair in the braid. This is the same technique as you use for hunter braids.
Braid until you are just above the ends of the hair. Tie off the braid by wrapping the yarn around the braid and tying a knot. If you aren't sure how to tie a braid off, this is an excellent article using a contrasting yarn colour for clarity. If your horse's mane is thick, it can be difficult to get the knot tight enough so for those manes I like to wrap a braiding elastic over the knot (make sure to gently pull the ends of the yarn out of the way).
If a minor imperfection develops in the second half of your braid, as has happened here when my horse shook her head, don't worry about it. That part of the braid will be hidden later on.
I like to braid the entire mane down before moving on to the next step to help me keep an eye on the size and length of my braids.
Take your pull-through and insert it down through the braid about 2/3 of the way down. Insert it straight down through the center (it's much easier if you insert it through a gap between cross-overs and that helps to keep the braid straight).
With the hook in the open position, put both ends of the yarn inside the loop and then close it. Bring the pull-through back up through the braid so that the yarn ends stick out of the front of the braid and then let the yarn fall out of the pull-through.
Gently pull the yarn straight down. Your braid should fold up and finish with the yarn pointing at the ground.
Now insert the pull-through straight down through to the back of the braid at the crest, as you would for a hunter braid. Grab the yarn again with your pull-through and bring it back up through the top of the braid.
Gently pull the yarn taut. Your braid should fold up a second time, with the folded bottom of the braid from the previous step ending up at the top of the underside of the braid. You might need to gently guide the folding process with your fingers as you pull.
Keep the yarn taut as you bring one piece of yarn around each side of the braid, tying a simple overhand knot under the braid.
Now bring one piece of yarn up around each side of the braid and tie another overhand knot on top. I like to use a surgeon's knot at this point, twisting an extra time to make it even more secure.
To finish, bring one piece of yarn down around each side of the braid again and tie them together underneath the braid, using two overhand knots to make sure it won't budge. Again, I like to use a surgeon's knot at this point. All of these passes above and below the braid serve to make the braid round and to corral any loose hairs into the braid.
Grab the two yarn ends and pull them out to the side or bottom of the braid so that you can cut them with the scissors without cutting the braid itself. There should be enough stretch in the yarn that they should disappear back under the braid when you let go of the cut ends.
Repeat the same steps for the rest of your braids until the entire neck is done.
To remove the braids, you'll need to cut through your final knot underneath the braid and then you might also need to cut your wrap knot from the initial braiding down.