To expound on jeffs description of "just the right distance", just the right distance is, well, sort of a felt thing, something that you'll come to know with experience. Whichever type of gap gauge you get, be it a gauge with flats, or sized loops, you'll look for the area of the flat, or individual loop, that is marked with the dimension you need. Slip the gauge between the electrode and ground. If you cannot slip it in, you'll need to open the gap, by bending the ground outward. Your gapping gauge should have a notch with which to bend the ground. If it goes in really easy and slops around you'll need to tighten the gap. Very gently tapping on a solid surface should do the trick, just check every tap or so.
Now for the "felt part. When the gap is right you should be able to insert the gauge, and as the gauge passes between the electrode and ground, you should feel the gauge just barely touching both points.
73 280 SEL 4.5