I do a lot of troubleshooting in commercial buildings.
#1 Don't outsmart yourself. ALWAYS check the simple stuff first.
Follow the flow of power.
#2 Listen to what they say ( and don't say ) Then check it all for yourself. Even if you have been assured that everything was done correctly.
#3 If you are stumped make a phone call and bounce a few ideas off somebody. Sometimes you will figure it out while you are describing the problem to them.
#4 Make sure your phone is charged before you call tech support........
#5 Be patient and use your head. They say that our brain separates us from animals, or was it our thumbs? Monkeys have thumbs and brains, I wish I had a monkey as a helper sometimes. Then again sometimes it seems like I do. I wonder if a monkey would have to wear a hardhat or if they would work for peanuts? I need to see how much a truck load of bananas cost, what if he has a nut allergy?
#5 Be patient and stay focused