If you have the 3-channel, see pages 11 and 12 of the manual.
For the 2-channel, see pages 9 and 10.
There is no simple answer. There is generally an FX send level and a return level. The send level is to set the guitar signal level in the FX boxes. Set it too low, and the FX boxes get no signal and do nothing. Too high and they overload. The correct level depends on your FX boxes and amp settings. Use your ear.
The FX return level is to set the output level from the FX boxes so if you switch the FX loop on and off you keep the amp output level constant. Or you can set it to boost, or whatever you like. Just switch the FX loop on and off and adjust using your ear.
MIX is important. This sets how much serial vs. parallel signal you get. If you are using digital FX that have both a dry and wet signal in them, you will want to set the loop control to 100% wet. If you run less than 100% wet you will get a comb-filter effect, as the dry signal in the FX box will have a slight delay, and then mix with the dry amp signal. For effects that have only wet, you can dial the Mix knob to taste.
