I bought official Nintendo protectors (also came with 2 styluses and a cleaning cloth) from Gamestop for a little less than $4.00.
I applied them as soon as I took my N3ds XL out of the box. A little less than a year later, everything is still firmly applied, but that 1MM gap is so you can take them off without damaging the screen while you would try to pry the screen protector from the corner.
The package also included a paper card to apply said protectors.
My best advice: wipe your screens first, then, use whichever dominant hand can make a controlled wave-pattern to squeeze out air bubbles. With luck, you should have no static attracting dust particles.
