Use a spade or shovel to break up the soil until it's loose and airy to a depth of 10 inches.
Choose the strongest cane on the grapevine. Remove all the others with pruning shears.
Remove the grapevine from the nursery pot and check the roots. If any are broken or significantly longer than the rest, trim them with the shears.
Plant the new grapevine in a hole wide enough to accommodate the entire root ball without any of the roots bending. Dig it the same depth as the vine is growing in the nursery pot. When you place the roots in the hole, spread them out with your fingers and then fill the hole with soil.
Cut back the remaining cane until it has two buds.
Tie the cane to the trellis using soft material, such as the green plastic tape available at gardening centers.