One of the best methods for grafting grapevines is T budding, according to Texas A&M University. T budding, sometimes called shield budding, is a process to which the part of the vine that is being grafted (the scion) is reduced to one bud and a tiny sliver of shield-shaped wood to which the bud is attached. This piece is then attached to a t-shaped cut into the bark of a grapevine. Grafting is a process that takes practice and patience to do correctly.
Cut the bud scions in early January when the vine plants are dormant. Position a grafting knife ½ inch below the bud and slice upward at a slight angle. Finish the upward slice at ¾ of an inch above the bud. Detach the shield with the bud on it by making a perpendicular slice into the wood above the bud.
Place the bud in a plastic bag with 1 tsp. of water. Store in a dark, cool place, such as a refrigerator, where temperatures stay a constant 40 degrees F.
Time the grafting procedure for early to mid-spring. During this time, the vine's sap will not flow profusely.
Select a point on the vine just above where a leaf, tendril or another vine emerges. Make a vertical cut into the vine just deep enough that the bark layer will easily peel back. Make a perpendicular slice across the top of the vertical cut to form a T shape.
Peel back the bark in the flaps where they form the T shape, creating a pocket. Slip the bud shield into this pocket and close the flaps around it. Trim the top of the shield if necessary. The wood of the bud sheild should not touch the bark above the T cut.
Wrap the polyethylene grafting tape around the bud and the T cut.
Remove the grafting tape in two to three weeks, after the union has healed. Prune away the top of the existing vine so that the bud will be forced to produce a vine in it's place. If the bud is grafted in summer rather than spring, the pruning will have to wait until the following spring so that the bud can overwinter safely.