This produces ends that are cut straight-across.
Start with combing your hair as straight as possible, removing every tangle. Comb the hair back and put it in a low ponytail. Hold your head straight. Pull the scrunchie as far down your ponytail as possible, and as close to the body as possible too. When you can't reach anymore, try to grab the scrunchie from behind your back instead. Pull it all the way down to the end of the ponytail. Maybe you'll need someone to help you with this.
Be careful to pull the scrunchie down in a strictly vertical manner. It's so easy to end up with hair a little shorter on one side
and longer on the other, just because the scrunchie was pulled a little to the left or the right. You might want someone to help you with this.
Update: I'm now tilting my head to look at the floor instead of looking straight forward. This produces even better results. If the head and the eyes are facing straight forward, then the shape will end up like a very slight inverted U with the sides a tiny bit longer than the middle. If the head is tilted so that you are looking towards your toes, the middle part will be even in length with the sides.
Now, bring the scrunchie at the end of the ponytail forward, over one of your sholders. Get hold of the scrunchie (be careful not to pull any strands while doing this!!) and trim the tip of the ponytail off. If your hair is too short to reach, you can ask someone else to trim the tip off. You can redo this step a couple of times to make sure that it's even and that you haven't missed any hair strands.
Step 2 (optional)
Part your hair (like when making two braids) and bring the hair forward. Hold your head straight, and trim 1/4 inch off the ends, just to even it up.
You can hold the hair either like this: Or this:
I prefer the first grip, it holds the ends firmly.
When you're done, your straight-across cut will look something like this: