When flying by reference to the instruments, you will develop a technique for scanning. It doesn't really matter how you scan. It just matters that you're able to take the information from the instruments and change it into a mental picture of what the airplane is doing. In order to do this effectively, you should be cognizant of a few common scan errors. It is far better to learn to scan the instruments in a way that avoids these errors, over learning a faulty scan and then going back to try to learn to scan a different way. Even if you are able to fly under normal circumstances with poor scanning technique, the scan errors might be brought out by an instrument failure or high workload situation.
The first error to be aware of and avoid is a rapid scan. A pilot having too rapid a scan looks at the instruments, but doesn't take in the information. Understanding the flight instruments is a skill. It's perfectly okay to be a little slow in the beginning. Develop a good scan and your rate will get faster with experience.
Another error is omission. In this instance, a pilot omits scanning one or more instruments. It might be that a pilot ignores one instrument during one maneuver, then ignores another during a different maneuver. Make sure your mental picture of the situation is not based on partial information by intentionally scanning each instrument.
A common error is emphasis. Here the pilot gives emphasis to one instrument over another. It is very easy to get suckered into spending a lot of time staring at the attitude indicator, for example. It's right in the middle and give you a direct picture of aircraft attitude. However, it could also break at any time. Don't wholly rely on a single instrument. In my way of flying, the attitude indicator is almost always a secondary instrument and should not take more focus than other instruments.
Another word on emphasis and omission: Primary instruments should not be emphasized over secondary instruments. Primary and secondary is merely a recognition that one instrument tells us the one thing we are using to control pitch, while another will tell us the information we need to control roll. The other instruments will be secondary, but not less important to scan. All the instruments should be scanned equally so that the pilot can maintain an excellent picture of what the airplane is doing at all times. Having this picture allows us to rapidly identify and correctly respond to the failure of a flight instrument or group of flight instruments.