Here you'll find the quick version of 'how to prune a grapevine.' These instructions assume that the vine has been trained competently in previous years; they are also applicable for a vine which wasn't trained properly but is still relatively young (2 - 5 years). With these guidelines you can start to bring your vine back into shape. Badly neglected vines, however, will require a more drastic method, called restoration pruning.
1. Recognizing the previous year's wood: This includes all shoots grown during the previous year. They are usually yellow-brown, ochre, or red-brown, and have buds at intervals of 5 - 15cm which, when scratched, reveal their green inside.
2. First pre-pruning: Cut ALL these thin, long canes from the previous year back to about 4 - 6 eyes (buds).
3. Counting the buds: Fruit for next year will only be produced from buds grown on these previous year's canes. How many of these buds are there now per square metre of wall space? You should definitely have more than 20 per sqm.
4. Selecting buds: 'Good' canes from the previous year are at least as thick as a pencil and are as close as possible to the thick stems - not at the end of metre-long thin branchings - provided that your vine has some kind of a recognisable stem framework. Now select about 5 - 7 shortened 'good' canes per square metre from the previous year, which are to be kept, and mark them (e.g., with colored ribbons).
5. Second pre-pruning: Cut off all unmarked canes flush with the next thicker cane, or cut them off completely right back to the main trunk. This should have removed about 90% of all branches.
6. Fine pruning: Prune the remaining marked canes back as follows: in the lower areas to 2 - 3 buds ("spurs"), and in the upper areas to 4 - 6 buds ("rods"), so that you end up with approximately 15 - 20 eyes (buds) per square metre of wall space.