Many jewelry makers will tell you to pull a piece of tail from the center of the tail out sideways and cut it near the base. It will not show once the tail is brushed back into place. You can do this if you like but there are a lot of wasted tail hairs this way.
The way that I have collected tail hairs is by pulling them. I would prefer that you use this method as it results in more useable tail hair and less damage to the tail. None of my horses mind me doing this. How many times have you gone into your horse's stall and found tail hairs hanging on something? Probably quite a bit. I need the hair to be about the size of a pencil in diameter. More is better than less. Make sure the hair is dry when you ship it to me. If you collect hair from more than one horse make sure you keep the different color separate so I can do different colored strands in your braid.
Make sure the tail is dry, clean and well brushed.
Pick up the tail by the bottom and pick out a few of the longest hairs.
Firmly but gently pull the hairs out. Do not do too many hairs at once. If the hair breaks it is because you pulled too many hairs at once.
The hairs you pull should be all about the same length using this method. Organize them into a mini-tail collected up at one end. Discard any short pieces.
Collect about a pencil sized rope of hair. Twist and put in an envelope to send to me.
Do not use mane hair.
Questions? Feel free to email me.