Many riders make this process way, way, way more complicated than it needs to be:

Think of your bike as having 3 gear ranges: One for steep uphills, one for fairly level ground, and one for either downhills or for the two times per year that you have a significant tail wind. Use your front derailleur to shift into whichever chainring matches your condition. Fine tune your gearing within those ranges with your rear derailleur.

That's all that you really need to know.