For a slight U-shape to the ends.
Start with combing your hair as straight as possible, removing every tangle. Part your hair in the middle, all the way down to the back of the head. Then bring the two sides of hair forward.
Comb each side carefully until every hair strand seems to be lying in the right direction. When you're done, tilt your head back as far as comfortable and then comb some more. You'll feel very silly doing this, and you'll look silly too. But just try to ignore that for a while!
Concentrate on one side of the hair at a time. You can use a scrunchie to hold the other one in place while you trim. Comb the un-scrunched side while still tilted back. Grab the ends of your hair with one hand (using your index and middle finger). Hold the scissors with the other.
When you have a firm hold you can look down on your hair again. Make sure that your grip is firm and avoid shifting any hair strands when you bend your head to look down. Then bring the ends up and carefully trim off as much as desired.
Note: For a deeper U-shape, look straight ahead in front of you when trimming instead of looking toward the roof. You can also look toward the floor for an even deeper U.
You can hold the hair either like this: Or this:
I prefer the first grip, it holds the ends firmly.
Trim both sides of hair this way, and try to make them as even as possible. You don't want to end up with hair that is shorter on one
When you're done you should have hair that is slightly V-shaped when brushed back. To remove the V and replace it with a U you need to comb the hair back and put it in a low ponytail. Bend your head forward to look toward the floor. 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 so that you don't pull it more to one side or the other.
Please note: The image is showing this step while looking straight ahead, but I have found that looking toward the floor produces even better results.
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 (this removes the V). If your hair is too short to reach, you can ask someone else to trim the tip off. Comb the hair back again and make sure that you're satisfied. If you've still got a V you'll have to ponytail your hair again and trim more of the tip off.
Step 4 (for perfectionists, not necessary to do, but highly recommended!)
If you're a perfectionist like I am, then part your hair and comb the two sides forward again, while holding your head up looking straight ahead (no tilting). You will notice that your ends are a bit uneven, so just even them up (1/4 of an inch or so).
Step 5 (also for perfectionists)
Finally you can comb all your hair to one side instead of parting it in the middle. You do this to make sure that you haven't missed any strands (This might happen if your part isn't exactly in the middle of your head). Snip off all strands that are longer than the rest of the hair, and make sure that it's even. Do this with the other side too.
When you're finally done, your U-shape will look something like this:
A deeper U-shape (head held up to look straight ahead instead of looking towards the roof in Step 2) on longer hair looks like this: