Pruning

With our old house, we inherited a massive grapevine. It had some grapes on it last summer but it was so overgrown and wild that the sun could not get to most of the new growth, therefore not producing much.

The literature that we've read on the subject tell us to prune when the vine is dormant (winter), but we can't seem to find any other directions that are in layman's (?) terms.

So today G started hacking away at it. It's got some big and sturdy vines, but we don't really know what we are doing. Are there any grapevine experts out there that could give us some help.
Now we can see right in the neighbors backyard :(