I don't want to say that you should always just tell someone to do what you say when you're paying them, because you might run into a situation where they're right and you're wrong. For the future, my best advice is to get them to take the time to explain their reasoning to you. For example, you show up with a diagram for how to drill, he says that's not right, and you don't let him put bit to metal until he can tell you why it's not right.
Just recently I took my Hyundai in for a problem. The service advisor called to tell me that I needed a $400 part...but he couldn't tell me how it would fix my problem. I politely declined. He got very frustrated with me, but his frustration is cheaper than a $400 part I might not need.