Vines are dormant at this time of year so they won't be damaged by hard pruning. Leave it later and they run the chance of sap bleeding from the cuts, which can weaken the plants.
The idea of pruning and training a vine is to form a basic framework of branches (known as rods). The shoots that come off these main branches are trained at right angles to bear flowers and eventually the bunches of grapes.
Prune the main stem back to the top of its support and reduce all the other sideshoots to one or two buds, this will encourage sideshoots to form. These sideshoots can be lightly pruned in summer.
This basic winter pruning can be repeated every year as the vines mature. You could train one, two or even three main branches to grow in a greenhouse or over an arbour. Reducing the sideshoots to two buds in winter keeps the framework neat and productive.
It really is that simple - but do it NOW!