[header] How to graft grape vine [title] Cut the top of the vine. [step] Select a fairly large stock vine and saw off the top roughly 30 minutes before you intend to graft the scions. [substeps] Note that this is the simplest grafting method to use and generally produces good results. 
You should cut the top based on the desired head height of the grafted vine. Make the cut at a clean, unblemished portion of the vine and cut until the height of the rootstock is roughly 4 inches (10 cm) shorter than your final desired head height.