Measure the size of the piezo and the jack enclosure. Cut two pieces of the double-sided foam tape according to the measurements -- one to mount the piezo element and another to mount the jack enclosure of the drum trigger.
Mount the piezo element with one piece of the double-sided foam tape on top of the guitar's bridge, which is the small piece of wood where the strings are attached to the body of the guitar. Mounting the piezo element to the side of the bridge, where the lower strings attach, provides a warmer, more resonant signal when it is plugged in.
Mount the jack enclosure to the body of the guitar with the second piece of double-sided foam tape. Mount it behind the bridge at a slight angle, so when a 1/4-inch instrument cable is plugged into the jack enclosure, it hangs downward.