Usually quite a few (if not all) plugs these days come pre-gapped...so the gap will already be pretty close to what you need...if not dead-on.
Just slip the right gap [of the tool] between the electrodes and you should feel just a bit of friction as you slip the tool in and slide it back out.
If it's too tight and the tool can barely slip in I gently rock the tool up and down to open the gap a bit more and try sliding the tool in and out again....be gentle, it's easy to open the gap too much.
There's no science to it.
12 Wrangler Sahara JK8 conversion, <10 miles (FOR SALE, $56,700)
01 WJ Limited, 4.7, 247, black on black, 157k miles (sold)
96 XJ Classic, 4.0, 242, black on grey, 267k miles (sold)
00 XJ Sport, 4.0, 242, fern green on grey, 76k miles (sold)
90 XJ Wagoneer, 4.0, 242, burgundy on tan, unknown mileage (sold)
97 ZJ Limited, 5.2, 249, black on black, 138k miles (wrecked 12/15/05)
98 ZJ Limited, 5.2, 249, burgundy on black, 197k miles (sold)