Identify the three kinds of wood that comprise the growth cycle of grape vines: old, fruiting and new. The major branches, or canes, support the vine and after three years are regarded as old wood. Their job is to support the vine and bring nutrients to branches that will grow and bear fruit. This year's new canes are next year's fruiting canes; fruit is produced on second-year canes. New wood is the strongest-growing, establishing the reach and size of the vine. How you prune new wood will determine future crops of fruit.
Cut old wood only to shape and control the size of a severely-overgrown vine. Unless old canes are dead or badly damaged, you will remove additional stages of growth when you cut them back. Use a pruning saw to cut dead or damaged canes back to ground level. This will reshape your vine, and the intent is to redirect growth-energy to sounder parts of the plant. This can be done in spring or fall.
Prune second-year or fruiting wood back with care. Buds on second-year spring wood are your source of fruit and also the source of new growth. This pruning should be done in very early spring, while the vine is still dormant. Use clippers or saw to bring these canes back to a nodule (the swelling that indicates bud formation. You will then be able to determine how to shape your vine in order to keep as many new, potential fruiting canes you can retain. Experts point out that fruiting wood typically produces far more buds than a vine can sustain through fruit production.
Use established formulas, cited in references, to determine how much to prune from potential fruiting wood. Experts counsel a general ratio of 30 buds to a pound of pruned wood.