As part of showing horses it is usually necessary to braid their manes for dressage, and if you have a long maned horse you'll need to use a french braid. There are two types of french braids, a three strand, and a four strand.
Here is my test subject, plenty of hair to work with.
This link from Lucky Braids shows you how to do the traditional 3 strand Running braid which is very common. It's very difficult to get this braid to stay on the top of the neck, and as you add in hair it tends to get longer and move down the neck, especially if you aren't very good at it. It will also come undone if you let the horse put its head down to graze or eat hay. Or I just suck at braiding. I've been experimenting with using 4 strands which is a traditional French braid, sometimes seen with the baroque breeds, as a way of keeping the mane on top of the crest.
Here's a closer look. The key here is to get a ladder so that you are standing over the top of the horse. You want to keep your LEFT hand on the left side of the neck and your right hand on the RIGHT side of the neck so that you keep the braid on top of the horse's neck. Pull the braid tight as you go.