There are complicated gear charts for using both front and rear to shift exactly from one gear to the next. I don't bother with them. I just shift on the back most of the time to maintain a reasonable cadence. If I am going up a very steep and long hill, I may use my small front ring as a bailout gear, or the big one when blasting down the other side.

As far as the numbers on the shifters, just ignore them. What matters is that you are pedaling at a comfortable speed and effort. So just keep it in the middle ring on the front and use the rear as needed to maintain a comfortable cadence.

The thing to avoid is using the extreme small-big gear combinations. The chain is then at an extreme angle and the derailluer might not be able to handle the extra slack or tension.