Step 5: Insulate the strings
Since the program figures out which note is played by putting a charge on each string in order, each string will need to be electrically isolated from every other string. Unfortunately, the all-metal bridge works against us. Also, the tension in a tuned guitar's strings tends to cut through a variety of possible insulators. But still, we persevere.
If you haven't already, remove each string from the body of the guitar. If you're not used to working with a guitar, it might make sense to work one string at a time, so that you don't wind up with a reverse-strung guitar.
Now slide a bead down the string. This bead must be large and strong enough to keep the brass ring on the end of the string from slipping down into the metal hole it normally rests in, insulating the brass ring from the metal bridge body. Extra points if the bead tapers, so that the bead rests in the metal hole.
In these pictures, I've soldered the strings to the diodes, although since then I've found it's easiest to attach the diode on the other end of the wire.
Now put the strings back into the body, but don't attach them to the neck just yet. The strings will still touch the top surfaces of the metal bridge, so they need to be insulated there too. Unfortunately, the tension in the strings tends to cut through a variety of materials.
The best solution I have found so far is plastic drinking straws. Cut an inch or so off a straw, then cut along it's side, so you have a rectangle of straw material. Holding the part of the string that is coming out of the front of the guitar, fold the straw piece over the string and wrap it as tightly as you can, the push it down into the hole in the bridge so that when you pull the string towards the neck, the straw covers both the area where the string comes out of the bridge and where it touches the bridge on the face of the guitar.
Now reattach the strings to the neck. Tighten the strings until they are firm, then use the multimeter to make sure that none of the wires are electrically connected to the others, fixing the insulation accordingly.