I have several muscadine and scuppernong grape vines on large trellises in an "espalier" type privacy arrangement around my patio and now that they are going dormant I'd like to get some info on properly pruning them.

First off, is when to prune. Next up is how?

Should I just take the tips off of the spindly growth or should I trim back to the woody branches?

I would like them to stay thick on the trellis and am more interested in their privacy aspect, the fruit is a secondary concern. Less fruit next year is less important than privacy.

Any advice and guidance would be appreciated.

Gary