Strap-Pin-Jack-Plugs are a easy and great way to have a jack without defacing your guitar with a new hole. You will need to remove the strap pin that is on almost every guitar and in some cases, widen the hole to accommodate the new jack-pin.
Next, we need to prep the piezo pickups. I like to glue a small amount of felt on the none wire side of the piezo. This will help allow the piezo to vibrate more and it makes any removal down the road a little easier. I wouldn't use any glues that are too strong; making this installment permanent.
The placement of the piezos on the underside of the guitar bridge should overhang off the bridge slightly. This should allow the piezos to vibrate more freely.
From here, we have some options on how we wired the multiple piezo pickups. I chose to wire mine in Parallel. Making each pickup run independently. You can however choose to wire the pickups in Series. This will make the pickups function in conjunction of each other changing the sound and characteristics of the overall sound. You may however run into resistance problems with this wiring. The overall volume output may be slightly decreased.
Other tips I've found useful in going this route in amplifying acoustic instruments. You most likely will need to run an extra ground wire from the jack sleeve to the underside of the bridge and through a bridge-pin hole to make contact with a few strings behind the bridge saddle. This will reduce any extra noise in the amplified signal. Using an EQ pedal or having the sound-person at the board roll off any frequencies that may be unpleasant, so you can customize your sound with the EQ. I think one of the best resources for buy cheap piezos is the site: cbgitty.com. You can get a 12 pack for about $10 and start amplifying anything you want. I've even used these pickups to plug in my acoustic foot stomp box drum.
Have fun, experiment, and try not to damage any of your vintage equipment. Here's a picture of a mandolin I recently did an installation on.