This was the method I learned when I first started to trim my hair. It seems to work especially good on wavy or curly hair.
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. Hold your head up as straight as possible while doing this (i.e. look straight forward and avoid tilting it to the side).
Note: Holding your head so that you look straight ahead of you while trimming will give you a 'medium' V-shape. For a slighter V, tilt your head back instead to look towards the roof. For a deeper V, look to the floor. You might want to experiment with this to find the perfect shape for you.
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 looking in your preferred direction as described above (straight ahead, to the roof, or to the floor). While still looking in that direction, 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 at your hair again. Make sure to have a firm hold so that you don't let the hairs slide between your fingers when you look down at it. Then bring the ends up for a better view and carefully trim off as much as desired.
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 side.
When you're done you should have hair that is V-shaped when brushed back.
Step 3 (For perfectionists, not necessary to do)
Finally you can bring all hair to one side (i.e. not parted in the middle) and then comb it and trim it following the steps above. 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. Then comb all hair to the other side too and do the same. Comb a couple of times from side to side if you're uncertain that it's really even.
Note: If this sounds confusing, please skip this step. Clearer pics will be added later.
When you're finally done, your V-shape will look something like this:
A slight V-shape on longer hair will look something like this: