When do you do it?
Typically this is done anytime after mid-June or so. I personally prefer not to do it until after my fruit is harvested to avoid sunburn to the fruit. However, if you have a late season variety this is not possible.
What do I do?
The object of pruning this time of year is to reduce the height and width of the tree, if necessary. You will notice that stone fruits do not bear on new wood but on older fruiting spurs. You will not be pruning off these spurs. To see some examples of how to do summer pruning from the experts at Dave Wilson, see the video below. Note that first 8 minutes are about espalier, so skip to 8 minutes if you prefer but it is full of good tips!
To read more about Summer Pruning aka Backyard Orchard Culture click here: http://www.davewilson.com/home-gardens/backyard-orchard-culture
Happy Summer Pruning!