Essentially, click the W in the track to turn WRITE automation on and be sure the R for READ is on. R should be on by default but you will have to turn W on.
With Write on, just move the fader up and down where you want the volume to change and Sonar will remember this and do it for you. (That is what the R does: reads the changes you made wile W was on).
If you don't like the changes, just play back the track and move the fader anywhere you want something to be different--you don't have to change the whole song: you can just change any part you don't like.
When you are done, you can click W to turn Write off if you want to so you don't accidentally make a change, but it is OK to leave it on.
If you temporarily want to hear a part without the automation changes, you can click the W and R to OFF. If R is off, the fader will stay in one place instead of moving as you programmed it. Sometimes I turn Write and Read off to see what the track would sound like at a different level without messing up the automation.
You can also highlight the whole track and go to EDIT | CUT and choose automation to get rid of the fader changes if you really mess up.
By the way, this automation function works with panning (left and write) as well.
WARNING: Be careful what you change when automation is on. Once I was fooling around with the EQ while the track was running and automation was on and I programmed in some pretty crazy EQ changes.
You don't have to do this in Console view. You can do it in Track view as well. People use Console to see all the tracks at once and their various pan and level settings, but the function works in track view as well. Good luck!