|mobil guitar tuner|
When you tune a wooden guitar, you tune up the tension on one string, you are at the same time changing the tension on the other strings whith is changing their tuning. So make several passes through the six strings, tuning each a couple of times, and last time more detailed and exact. Ether you are using an electronic guitar tuner or by ear.
If you use nylon strings for classical guitars have in mind they take longer to stretch and settle into tune.
After you restrung your guitar its good to grab each string around the 12th fret and give it a few yanks, straight up and away from the guitar. To stretch them by hand right away than to have them stretch and drop in tune later which is what new strings do.
Old guitar strings get flattened where they contact the fret and become difficult to keep in tune so change your guitar strings often.