The process is pretty simple—first, make sure you have an actual toothpaste. Gel-style toothpastes won’t work for this. You’ll also want a damp cloth handy. Clean the glass thoroughly first, then rub a dab of toothpaste (they say “dime-sized,” but I think that depends on the crack in question and the size of the object you’re trying to fix) on the crack. Rub the toothpaste in firmly in a circular motion (but obviously not so hard you risk damaging the glass) for about a minute. Look at the glass, and if that did the trick, wipe the excess toothpaste away with the damp cloth. If not, repeat. That’s about it.
Obviously this isn’t a catch all, and we wouldn’t recommend it on expensive glasses or glasses you can (and should) get a professional to repair if they’re damaged, but if you’re short on cash or you have a pair of readers or sunglasses with those tiny annoying stress cracks in them, or a piece of glass furniture or glassware with a few scratches on them that you just can’t help but notice, it’s worth a shot.