My grapevine ("Boskoop Glory") is trained accross the top of my greenhouse. I cut all last years side shoots back to the main vine mid-winter. Now it's time for it's spring prune. Each group of side shoots should be roughly a foot apart, any that aren't I snap off where they join the vine. Then I choose the best two from each group of side shoots and snap of the other weaker ones. Then I cut back each side shoot at two leaf joints above the little developing bunch of grapes:
This pic probably shows it better, you can see how the side shoots are cut back two joints above the little developing grape bunches:
Any new growth from the leader and side shoots of these side shoots needs to be pinched out and any new shoots trying to emerge from the main vine needs to be rubbed out too. The vine will try to produce lots of these so it needs to be done every week to ensure the energy of the vine goes into the developing grapes rather than superfluous growth.