It's not the good video.
Press Edit on Monitor section
In the screen, select Phones, or Monitor
Affect to a mix, for example, Mix 1
Press Mix 1 on the left of mixer
You can adjust the level for each channel you want to ear in headphones or monitors
Press Mix/Fx Master on top of users functions, on the right section, you have the fader of the Ax1, in it is the main volune for Mix1