Start tuning your bass by plucking the top string. It is the first string you see as you look down on your bass; the string is also known as the E string. When you pluck the string, the tuner shows if you need to tune the string up (too low) or down (too high). Pluck the string and adjust the peg as needed.
The string is in tune when the "needle" hovers in the middle of the tuner window. Repeat for each string on your bass; that's it, your bass is in tune.
You can access the tuner from the pause menu of a song (both Practice and Play mode) and from the main game play screen.
When you access the tuner, the Auto mode is toggled green. This means that when you pluck a string, the tuner suggests which string you are tuning. Tuning in manual mode can be useful if the strings are too out of tune, or if you are tuning new strings for the first time.
To start manual tuning, select a string on the tuner screen. When you do this, you hear what the tuned string should sound like. Pluck the string on your bass and adjust the peg as needed. Repeat this with each string to tune them.
With the Audio setup button, you can access the Advanced audio settings in the Game tab.
If Yousician can't hear you at all, check troubleshooting tips here.