You must understand the system before you can begin debugging it. Here is a novel thought. Read the instructions. This means you should go over the specification for the system. You can also read source code comments. Review the design documentation to gain understand.Now you must understand one other thing which reading the documentation. You need to take what you read with a grain of salt. Don't trust everything you read.
On the other hand, you really need to know the contents of the user manual. Read it many times until you know it well. You must have a firm grasp on the fundamentals of the system. When I come back, we will get to the second rules of debugging - Make It Fail.
