Look over the grapevine in mid-summer and find a cane that is this season’s growth.
Bend that cane to the ground. Make sure that it's long enough to lie on the ground, leaving the tip free.
Score the stem in four or five places with a sharp knife where the branch touches the ground.
Mound soil over the cane, leaving the tip uncovered. Lay a brick or push down two U-shaped wires to hold the cane against the ground, if the cane is unruly.
Keep the ground moist so the cane will produce roots. The sections where you scored the cane will produce roots.
Sever the cane from the parent in the fall when the grape goes dormant and roots have developed.
Dig up the rooted grape and plant it.