This is a fantastic trick I found for cutting long hair all by yourself! I came across it on a long hair forum while googling how to do it. I've been cutting my own long hair for about a year now, and I love it. Plus, it saves quite a bit of money depending on how often you get a trim. Keep in mind that I can't recommend it for curly hair because I have the straightest & thinnest hair ever.
I also can't promise this is the most precise method, but with a little care you get your hair nice and straight. :)
And big thanks to audreyobscura
for photographing this and supplying gifs!