How to put on a NATO strap

Install the spring bars with a spring bar tool or a small screwdriver like the ones used to adjust eyeglasses

Once installed, you can now put on you NATO strap

First, remove the long end from the keeper

Then, slide the strap downward through the two spring bars

Take the long end again and slide it through the keeper

You can now fasten the strap as you normally would and finally tuck the tail inward or outward (your choice)