Originally Posted by echinosum

Grapevines need (i) structural formation and (ii) annual systematic pruning.

Overview - you need to establish a core network of main branches, which you should in general support on wires or other supports. This should be fairly simple. One central stem with two or three branches on each side supported on 2 or 3 wires would be a very basic formation. There are also systems with 2 main stems and parallel wires. Having formed this basic structure, you then prune all of the new growth right back to the basic structure in the dormant season, say around Feb, each year, though being careful not to damage the incipient buds at the joints. New shoots readily form from these joints in the old wood.

However you probably don't have a good basic structure at the moment. It is important not to cut any of the thick woody branches during the growing season, as it can result in weeping. So what you do is, while the green stems are reasonably pliable in the growing season, is choose some vigorous shoots that you are going to make your main branches, and train them along wires, tying them in. You may even be able to rearrange and select few now, though they get a bit stiff later in the season and may break if you arent' gentle. Then having chosen your structure, wait till winter, when its fully dormant, and cut everything else off. If you have some bits missing, or its not quite right, you can train some more shoots where you want the next season, much easier to tie them where you want to go while they are soft in early summer, and then keep those rather than cut them back.

Then for good production and not collapsing under its own weight, you probably need to do a bit of summer pruning. You can always cut through new green shoots in the summer without risk of bleeding. Since grapes are only borne on about the first 2 or 3 nodes of any new shoot coming out of the old wood, you can always cut a new shoot back to about 4 to 5 nodes. Those new shoots may then also produce side shoots, which won't fruit at all, so you can cut those back to just one leaf. And then, in our cool climate, it is a good idea to cut off leaves that are hiding the grapes from the sun. Once you have seen how well the grapes have set after pollination, you may want to thin them out to a reasonable number as they are more likely to ripen if there aren't too many. Vines are often very vigorous so you can end up doing quite a bit of management to get a good crop. It's been a very bad year because of cool weather at the wrong times, and you'll be lucky to have ripe grapes.