Well, the best answer is practice. Bummer, but there it is. Make sure you tape the stencil down (hubby holding it is great, but it's still going to wiggle) and pounce in an up & down motion around the edges. You can swirl lightly - but try to swirl from on top of the stencil toward the middle (so you don't accidentally get under the edge).
Also you could go around the letters with maybe a paint pen to help neaten the edges? It's worth a shot.
I have a ton of stamps so that's how I would do it, but you'd need a crash course on ink types to use. You can get an alpha set for fairly cheap at Hobby Lobby/Michaels using a coupon and then you'll need a couple other things to complete your project (ink, etc).
Good luck!