In my garden the garlic is about two weeks ahead of the normal time to cut scapes, which means you may be ahead as well, so we'll be harvesting earlier too.
Cut a couple of inches above where the scape grows from the plant, there's really no major right or wrong, just cut enough so you'll have plenty of length to saute or grill. The scape is fairly woody (stiff) closer to the plant so that's why I cut a couple of inches above that part.
Below are links to a couple of past blogs on garlic so click for more information-
Glorious Garlic Scapes
Garlic Harvest - Fall Planted
Also, check out the video I did a few years ago with the Denver Post on harvesting garlic. If it doesn't open, please click on the link below.