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!
Step 1: What you need + tips:
- a scrunchie
- a pair of scissors for hair
- a brush
- a handheld mirror
I like to cut my hair right after I've washed it and it's totally dry. You want your hair to be dry and free of tangles when you cut it. :)
It's also a good idea (especially the first time you do this) to employ a hand held mirror and large wall mirror so that you can see how what you're doing in the back. :D
Step 2: Part your hair and make a low ponytail
Then use the scrunchie to tie your hair into a low ponytail - right at the top of your neck. Make sure this is in the middle of your head!
Use your hand mirror to check it. :)
Step 3: Pull the scrunchie down your hair
If your hair is especially long, you might want to pull from the bottom as shown in the second photo. This method is a little trickier, and I don't do it often, but it's doable.
Leave 3-4 inches of hair below the scrunchie.
Step 4: Bring the scrunchie around the front and cut
If you sense that it's shifted, you might want to give it another go. The hair that you'll be cutting should look fairly straight across - though not perfect because hair is always growing at different speeds!
A good rule of thumb here is to cut very little at a time. 1/2 inch tops! That way you can check it in the mirror to make sure you've done it right before you proceed. :D
Hold the ends of your hair firmly between two fingers and cut straight across.