IMO, besides the fact that every submissive is different, the reason why there isn't any manual on Dom conduct is that Doms are a law unto themselves. They behave towards their submissive however they damn well want to behave, within the boundaries of consent of course. If I were to find out that my Dom was simply acting how he thought he should or how he thought I want him to, I would be devastated.

Which brings me to your question: How to sound commanding? How to deal with a sub who needs to test her limits and your power? The most important issue here, I believe, is your mindset. You need to WANT to put her back in her place, to teach her that it is not her place to test you in that manner. Once you can overcome that and feel completely in charge, I believe the rest will follow. Simply following advice like 'you must have a stern voice and look her straight in the eyes' just won't cut it if it doesn't FEEL natural to you, regardless of what your opinion of your type of dominance is.