I'll get the ball rolling. By "MIXER", I assume you mean an external mixer (as opposed to the mixer tab in Mixcraft?)
I use an external mixer and the way I do it is as follows.
My rule of thumb is to adjust levels from the start of the signal path, forward. So whether its a mic or guitar you have plugged in, play (or sing) a passage at the loudest volume you're likely to use in the song and adjust the gain or trim knob on the mixer so that it only peaks or illuminates very occasionally. My mixer goes into my audio interface so I would then adjust its input level in the same manner - just avoiding the clipping light. Maybe yours has AD/DA converters and goes straight to your computer - that's fine. Now arm your track in Mixcraft and check the loud parts aren't pushing the meter into the red.
More important though is the master track meter (on the right in the mixer tab). As you add tracks, they sum to the master and its possible with a few tracks, that the summed master starts pushing into the red. That you want to avoid at all costs. Adjust by lowering the master fader.
The beauty of digital audio is headroom so you don't need the level running "hot" or approaching 0 db. I try and keep mine between -12 and -6 db for the peaks. If you like loud, turn up your monitors
Well, that's my 2 cents worth, let's hear from the others.
Hope this helps!