When is the right time to prune grape vines? What is the best pruning technique?

Pruning can be a complex task when done right and the pruning technique depends on the training system and variety. Following are the basics. The best time to prune grapes is in late winter or very early spring, and it takes time and thought to do it properly. Before you begin, look closely at the vine and identify the canes that were last year’s new growth. These are the canes that will produce the next crop. You can tell them apart from the older ones by the color and integrity of the bark covering. The bark of the newer canes will be brighter in color, smooth and tight, while on the older canes it will be more grayish brown and coming loose in papery strips. Pruning can be done to spurs or canes. There are numerous videos demonstrating grapevine pruning:
Pruning of Wine Grapevines
Pruning of Concord Grapes
Spur Pruning Basics