Summarize the following:
This is a quick and easy method to keep your horse's mane tidy for everyday training or riding, but should not be used for competitions where style is judged. Pick up three swathes of mane of any size, at the top of the mane. Start to braid or French braid as you would normally, weaving the strands over each other. Stop after the first weave or two, and use the steps below to braid the entire mane with a quick and easy method. Each time you pick up the section on the side nearest the rest of the mane (left, if your horse's mane falls on the right), add another chunk from the rest of the mane. Just keep braiding until you're at the end of the mane. You should have a diagonal braid running from the top to the bottom of the mane; it's fine if there is loose hair above the lower end of the braid. Finish by wrapping an elastic band tightly around the tip of the braid.

Summary:
Divide the top of the mane into three pieces. Begin braiding. Bring in more mane as you braid. Wrap elastic around the end of the braid.