No more painful to them than getting your own hair or fingernails cut. It does insult their dignity while you're doing it, but they forget about the insult as soon as you let them go. It's super easy. I choose a clean patch of lawn and kneel down on the grass, grab a hen, and hold her between my knees. Then I spread one wing with my left hand, and use the scissors to clip the flight feathers on the wing. You might want to look at a drawing to see which feathers are the flight feathers. It's the first 9 - 11... I'm not sure how many there are, but when you spread the wing, you'll see what I'm talking about. If it's your first time, you might want to have some corn starch or an other clotting agent handy just in case you clip a blood feather (a new one that still has blood in the shaft) or cut too close. After you've done one wing, you'll be an expert with all of the rest! Be conservative with your first cuts, and you'll be fine. You can always take a bit more off if it doesn't do the trick. After the hen tries to fly a couple of times, she resigns herself to staying on the right side of the fence. My girls can still fly up to their loft with clipped wings (you only need to do one wing) and still do the wing flapping/running to gain speed when running for the treat bucket, or running away from something, but they give up on escaping the fence.
Suz: I'd recommend that you try clipping a single wing on your most flighty bird. You'll see how easy it is, and decide that it's easier than chasing escaping girls all the time.