Power meters are a great training tool to help improve your cycling and achieve your goals however they can be complicated and daunting when you first get one and suddenly got all this data at your fingertips!
After reading a number of books and experiencing it first hand then what I would suggest is that you just fit it on your bike and just display the numbers on your Garmin without worrying about using it
as a training tool. Just experience the data that it is showing you and how it relates to your effort/terrain/heart rate etc.
I would set your power display to “3sec” rather that actual as this will stop the number jumping all over the place and will enable you to try and maintain a constant power level. It is also useful to display your “normalised power” figure which will be the average over the ride and gives you a more realistic figure that traditional “average” as this takes away the peak/troughs of pedalling on the road.
Once you have started to get used to the numbers then its time to start using it to its full potential! The first thing to do is work out your Functional Threshold power (FTP) as this is the base of all your training sessions and then work out your training zones.