When you're going to play your bass guitar with a band or another instrument, you need to be sure that you're all tuned to the same reference pitch. Using an electronic tuner is by far the easiest way to tune your bass. Modern tuners have a display that lets you see exactly where your string is (pitchwise), whether it’s sharp (too high) or flat (too low), and what note you’re closest to.
Buy a tuner.
Okay, okay! Maybe that goes without saying, but keep in mind that you want to use a tuner that can register the low bass frequencies. Not all tuners can hear bass notes well.
Plug your bass into the tuner via the cable.
This is the same electric cord that you use to connect your bass to your amplifier.
Strike an open string and let it ring.
Remember, low frequencies travel very slowly, and the tuner needs time to read the note.
Tune the string until the needle (or light) of the tuner is in the middle of the display, indicating that the string is in tune.
Make sure to check that the pitch indicator shows the correct note for the string (E, A, D, G), or you may find that the G string is in perfect tune with G#, which is way out of tune with what the G string is supposed to sound like.