Summation
Ok, let's sum this all up.
The shift lever or twist grip on the left hand side of your handlebar shifts the front derailleur allowing you to choose one of three ranges to ride in. The one on the right shifts the rear derailleur allowing you to make small changes within each range.
If you feel like you're pedaling too hard, use the right hand shifter to switch to an easier gear (like from 7 down to 3 on your shift lever indicator). If you feel like the pedaling is too easy or you're pedaling too fast, use the right hand shifter to switch to an harder gear (like from 4 up to 8).
If you're riding in a hilly area, use the shifter on the left hand side to switch to the smaller (inside) chainring on your crank (position 1 on your shift lever indicator). If you're riding on a smooth flat surface and at a higher speed, use the left hand shifter to switch to the larger (outside) chainring (position number 3). For normal riding shift to the middle chainring (position number 2).
That's about all there is to it. Enjoy your ride. Article - Gearing - Roberts Cycle

I know this is general but it explains what you are trying to do with your gears when you ride a bike on a everyday basis. Notice it does not mention gear ratio,number of teeth, etc. It just mentions how you change gears with the shifters for each hand and situation. When I asked a question a couple of months a go about this and I used similar numbers to ask my question. I was treated with ridicule like I did not understand what I was saying. Here is article that summarizes using the same numbers I was. My point of making this thread is when someone is asking a question there is no need to give complicated answers when simple direct ones will do.