So, you got a 12 strings acoustic guitar and not sure how to tune those 12 string acoustic guitar up or want to re-string your 12 acoustic guitar but don't know how to do it properly. Just follow me.
Before getting your tuner out to tune your 12 string acoustic guitar, please keep in mind that there should be 6 pairs of thick and thin strings on a 12 string guitar, you might also get confused what tuning key to turn as well but you will get used to it soon enough. Also note that when i say Low E it means just E nothing more.
There are many other guitar tunings but what you about to learn is a standard tuning for 12 string acoustic guitar, no tune down or anything.
Now, to tune a 12 string acoustic guitar, go get your tuner (every musician should have one) and just tune the main 6 strings to ordinary tuning: E A D G B E low to high, start tuning the 1st string from (the highest sound string) to E.
Next, skip the thiner string and you find the 2nd string, tune that to B and skip another thin string and tune it to G > A and Low E. Just keep skipping those thin strings and you should finished your main 6 strings in no time.
Here comes the tricky part, to tune 12 string guitar on the rest of the strings, let's start from the high E the tuning key should be on the right hand side if you hold guitar head up with fretboard facing you.
Just tune those first 2 thin strings to high E and high B. Don't worry if they still a little out of tune, go through other strings so they are stretch equally. Next, go to the 2 tuning keys on the top right, on the top tune this to G one octave higher than normal. And the rest strings are as you can see in the picture.
Tuning 12 string acoustic guitar tips: The lower G string is the weakest string. To prevent this, be sure to use the proper gauge of string specially for your guitar and never over-tighten. When you're not going to play it soon in a few days you should tune all the string down to prevent neck from go out of shape. In case the strings are too tight, you can relieve them by tuning them all down half step or one step to get the lower tone too.