Open the mixer window with the menu, by choosing 'Window > Mixer'. As mentioned earlier, it can be convenient to put the mixer window on another monitor or virtual desktop.
Set all of the faders to 0 dB. They are probably already set to this level, unless you changed them earlier.
Take the quietest track, when set to 0 dB, as the limiting factor on how loud the other tracks should be. Since it's generally safer to avoid amplifying audio signals, if we use the quietest track as the "base-line," then we'll have to adjust the level of the other tracks down to suit. In this case, the voice tracks are the quietest.
At this point, it's best to stick with adjusting the busses' faders. If you adjust the faders on the tracks, this will affect the panning, and could lead to confusing problems later.
Play through the session, and adjust the faders of the busses so that all of the tracks can be heard equally well. Remember that you're just aiming for most of the session to be balanced at this point; a single fader setting is unlikely to be acceptable for the entire session.
You can adjust the fader setting in two ways:
Click-and-drag the vertical, dotted control strip to the left of the level meter (which lights up as a track is playing).
Use the indicator box as a text field: click in the box, erase the number that it shows, and write in a new number. Press 'enter' on the keyboard to set the new value.
You might wish to change the order of tracks and busses in the canvas area, which will change the order in the mixer window. Putting all of the busses together makes it easier to see them.
You could also choose to not display the tracks, again allowing you to focus on the busses that you will be changing. Do temporarily hide a track or bus in the mixer window, use the toolbox on the left side of the mixer window. Un-check the "Show" box for each of the tracks or busses that you want to temporarily hide.
The "maximum level" indicator on the fader tool might help you to judge how loud each track is. This indicator is located above the meter, underneath the "Solo" button. The indicator displays the highest level produced by the track since the indicator's last reset. You can reset the indicator by clicking on it.