Yeah, NATO straps are excessively long and tend to be bulky on the wrist. That's how they're supposed to be and how they look most right: They're intended to fit over military clothing, and that origin is a big part of what makes them cool and popular today. So, before you get all gung-ho with your scissors, at least try tucking the long end back into the keeper and see if you can live with it.
In other words, if you're trimming your NATO strap because you want it to wear like a normal strap, you're kinda missing the point of a NATO. Further, unless you're a professional NATO-maker, most trimming jobs end up looking passable, but can also look shabby if not done perfectly. You've been warned.
Sometimes, however, the width of your wrist and the length of the NATO might conspire to leave you with just the wrong amount of length left over, such that it doesn't tuck in easily. That's an example of a reasonable cause for trimming. If you must mutilate your nylon NATO strap, at least it's quick and easy to do, and you're not risking ruining a major investment. There are many possible methods, but here's a simple one:
The following techniques involve blades and fire. Be careful and proceed at your own risk.
What You'll Need to Trim a Nylon NATO Strap
1. Determine and Mark the Desired Length
It's best to do this by putting it on your wrist so you know exactly where you want the strap to end. Make a mark with your Sharpie.
2. Secure the Strap End
The key ring is going to help you get an evenly rounded shape to the end by slipping it between the ring's wires which should hold it firmly in place. Some people use two coins to sandwich the strap and secure it with clamps. This part's all up to you, though: if you want a straight or angled cut, say, it's still advisable to secure the strap.
3. Cut the Excess Strap Length
If you're using the key ring, go ahead and apply some pressure with the thumb and forefinger of your non-dominant hand. With the other hand, use the blade to follow the curve of the ring (or coins) to shave the extra length off. Then go back with the razor and get any spots that look uneven or strands left sticking out.
4. Apply Fire
Keep the strap end in the key ring or coin sandwich because this is holding the fibers firmly in place. Lightly singe the ends with a flame.
Most NATO straps are made of nylon, which can be melted, and singeing the ends of the fibers will stop them from fraying. Be careful not to burn too much or you'll end up with a blackened look (also, be careful of your fingers, for god's sake, Billy).
Butane-based torch lighters (the kind of lighter with flames that shoot out rather than flicker like a candle) are best for this. Finally, push the freshly singed fibers down to help them meld together and create a more even look. (Don't do this immediately after melting them, as it'll hurt like hell. Wait 20 seconds or so, or use a surface such as a cutting board covered in tin foil to absorb the heat and any excess nylon.)
Your shortened NATO strap is complete.