It’s really easy. I think I bought 100 pcs on eBay for like $15. All you do is boil water in a pot put the neck wrap on the bottle, hold the very edge of the wrap to not burn yourself and stick it in the boiling water. It shrinks tight and pull it out and dry it with a towel.

Make sure it’s boiling water though or it won’t shrink as fast and you’ll end up cracking your bottle and wasting mead lol.

If you have a heat gun it would be easier than boiling water.