The tips posted above are great tips.
Keep in mind:
Once you have the braid in don’t let your horse stretch its head to the ground to graze. This loosens the braid and can rip parts of the mane out.
If you have spots in the braid that look like they may come loose you can take like 4" pieces of yarn thread it through and tie it in a knot, then trim the yarn. For the “Main-Ring” Arabian hunters we would use white yarn in the manes and tails and black for the white or grey horses.
Hair spray is def. your friend. The smaller pieces of mane you take the better, and tighter the braid will be. I pull the braid tight after each time I add another section.
Practice makes perfect!
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wanted to see your horse!