The 12-string guitar is oftentimes a more finicky beast than its six-string cousin. With a dozen strings to manage, it can overwhelm even the most proficient player (strings can go out of tune and even break). Properly stringing up the 12-string guitar can help prevent future performance problems.
A 12-string guitar utilizes 12 strings that comprise six different notes that are played by pairs of strings: Two low E strings, two A strings, two D strings , two G strings, two B strings, and two high E strings (from the lowest string to the highest). Each note is played by two strings that are tuned in octaves, a bass string and a treble string. For instance, the bottom E strings consist of one bass E string and one treble E string that is tuned an octave above it. The bass string in each pair is thicker than its higher-octave sibling. The strings are tuned treble to bass, from the lowest to the highest strings (except for the high G and E string pairs, which are not in octaves).
Remove all 12 endpins from the bridge of the guitar. Find the low treble E string and locate the ball of the string on the bottom end. Place the ball inside the first hole on the bridge of the guitar (the left-most hole if you are staring at the guitar standing upright). Replace the endpin into the hole, pressing down on the endpin while pulling up on the string until the ball touches the end of the endpin. The string should fit snugly in the hole with the endpin inserted fully.
Run the string over the saddle of the guitar, the thin piece located directly above the bridge. A saddle typically contains small grooves where each string rests. Find the groove that corresponds with the low treble E string and fit the string onto the groove (The string will more easily rest in the groove once you begin tightening the string. See Step 4 for details).
Take the top end of the string and thread it through the hole located on the first tuning peg (the bottom-most peg on the left if you are staring at the guitar standing upright). Keep threading the string through the hole until you have just a little slack left in the string. Then, wrap the end of the string clockwise around the peg and underneath the slack part of the string. Pull the end of the string up so that it tightens against the peg and the slack part of the string. Rest the string in the left-most groove of the nut (located at the base of the headstock).
Begin rotating the tuning peg counter-clockwise with one hand while holding the string in place on the peg with the other. The low treble E string will begin wrapping around the tuning peg and the slack will disappear as the string continues to tighten. Keep winding the tuning peg until the string sits loosely in the saddle and the nut on its own. Repeat Steps 2 through 5 for the remaining strings, beginning with the bass E string companion to the treble string you just installed.
Once you have strung all 12 strings loosely onto the guitar, begin fine tuning each string up to standard tuning with the help of an electronic guitar tuner. For example, turn the tuning peg for the low treble E string with one hand and pluck it with the other until it plays a low E tone (82.4 hz, or a minor thirteenth below middle C). Most electronic guitar tuners will indicate when you have reached the right pitch. Then do the same for the bass E string, tuning it exactly one octave below its companion. Repeat those steps for the remaining strings.
Once all 12 strings are in tune, take a pair of pliers and cut off the loose end of each string at the tuning peg, leaving only a centimeter of string exposed. With your pliers, bend the end of the string down so that it does not cut you when tuning in the future.
When restringing a 12-string guitar, it might take a few minutes to get it fully in tune because the strings stretch. When performing your final tuning, pull each string up gently away from the body of the guitar to help stretch it out. Also, play the strings vigorously once you've tuned them and then check their tuning against an electronic tuner again to ensure they are fully stretched. It is fairly easy to keep a guitar in tune without the use of an electronic tuner if you use standard tuning. The fifth fret of each string is the same note as the string above. This is a little more tricky on a 12 string guitar since you have pairs of strings for each note. However, when each pair is in tune, then you can compare the fifth fret of one note to the open (unfretted) tone of the string pair directly above it. The only exception is the G: Play the fourth fret of the G strings in order to arrive at a B tone, which are the strings directly below them.
Do not tune each individual string all the way to its respective note until all the strings are placed on the guitar. Wind the strings just enough so that they rest loosely along the guitar. Once all the strings are wound into the tuning pegs, you can fine tune the guitar. Be extra careful when tuning the treble G string: It is the one most likely to break during the tuning process.