RTFM (
Read The Fantastic Manual). Each sound board product is unique, particularly the digital ones, so contact the manufacturer for references to instructional material for your specific product. Some manufacturers are more helpful than others, and a few have DVDs or online videos.
Some manufacturer's will even have a local sales representative come by and personally teach you. If you can, record the training session for future reference. Place the camera up over the board looking down at it so people and hands aren't in the way of seeing what knobs are being accessed.
If your question is more general about "How to mix audio" or "How to do sound for theatre", then search those terms for media. The book: "The Sound Reinforcement Handbook" by Gary Davis, and the book "The Ultimate Live Sound Operators Handbook" by Bill Gibson are both good places to start.
Digital audio mixers definitely require you to up your game. As mentioned in the book
Zen and the art of motorcycle maintenance, you have to get in the right mind-set to learn this. The author quotes from an assembly manual for a children's bicycle:
"
Step 1. To assemble Japanese bicycle requires great peace of mind."
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Erich Friend
Theatre Consultant
Teqniqal Systems