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 it, the Auto mode is toggled green. This means that when you pluck a string on your bass, the tuner suggests which string you are tuning.
Tuning in manual mode is useful if the strings are too out of tune, or if you are tuning new strings for the first time.
Tuning in Auto mode
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 or down.
If the tuner shows that you need to tune down, turn the tuning peg for the E string on your bass towards you. If the tuner shows that you need to tune up, turn the tuning peg for the E string away from you.
The string is in tune when the "needle" hovers in the middle of the tuner window. Repeat with each string; that's it, your bass is in tune.
Tuning in manual mode
To start manual tuning, select the E string peg on the tuner screen. When you do this, you hear what the tuned string should sound like.
Pluck the E string and the tuner shows if you need to tune the string up or down. If the tuner shows that you need to tune down, turn the tuning peg for the E string on your bass towards you. If the tuner shows that you need to tune up, turn the tuning peg for the E string away from you.
The string is in tune when the "needle" hovers in the middle of the tuner window. Repeat with each string; that's it, your bass is in tune.
If Yousician can't hear you at all, check troubleshooting tips here.