Electronic tuner
The easiest and most accurate way to tune your bass is by using an electronic tuner. An electronic tuner allows you tune each string individually to the tuner, by indicating whether the notes are sharp (too high) or flat (too low).you can plug your bass directly into the tuner using the same lead that plugs into your amplifier
Relative Tunning
Here's another way to tune your bass. it is less accurate,as it assumes that your starting string is in tune and it is harder for the ear to