Tuning the board is the first step to getting a good mix. Start by setting the pre-amp gain for each channel, then get a good blend, and finally make sure it sounds good in the house.
Here is what a fader strip looks like:

Here is what the shared processing controls look like:

For each input, while the instrument is playing or the singer is singing:
For the singers, you can assign a name to the channel.
During practice, press the LR button and pull its fader all the way to -Inf. You’ll still be able to hear the band, because the monitors at the front of house are pretty loud. The monitors have a mix that’s driven by the band, so the relative volumes might not be what sounds good to the congregation. Raise the level of the LR fader until you can hear it kick in. You should be able to hear it around -5dB. I usually try to keep the main mix at about -3dB to 0dB on the meter, which is usually about -5dB on the fader.
At least once during practice and once during the service, walk downstairs and check that the mix sounds OK. I like to listen from the middle of either the left or right side.