How to protect a large piece of side territory
The answer is: Don't, unless you really have to.

The situation shown in your question is already solid territory. In this case, adding any inside stone to "protect" your territory further will reduce your score.

You should try to decide if there is anything that can be exploited in your area. Let's see how it is in your case:

$$ Initial position
$$ .............
$$ .....OOOOOOO.
$$ ..XXXOOXOX.XO
$$ OOX..XX.X.X.O
$$ OX.....X...X.
$$ XX.........X.
$$ X..........X.
$$ -------------


As a simple rule of thumb: Imagine your opponent playing all inside forcing moves in sente, then try to find the vital point. Is there a way to live?

$$Wcm1 Is there a way to live? Even if black plays very simple, white cannot do anything after B10.
$$ .............
$$ .....OOOOOOO.
$$ ..XXXOOXOX.XO
$$ OOX.0XX6X4X8O
$$ OX219..X537X.
$$ XX.........X.
$$ X..........X.
$$ -------------


If not — as in this example — do not add stones. If you are unsure, it is good practice to still play away (this is vaguely related to the concept of kiai). As sole exception, if you're far ahead and it's an important game, well, I suppose you may add an inside move to make 110% sure.

Of course, sometimes you do have to defend without question. Usually, you should try to defend actively, by moving out or attacking a weakness of your opponent's stones to make your own stones more secure. If that is impossible, simply add a move at the vital point.

Where is the vital point? Shape sense will tell you. Around 10 kyu you should see spots that defend most or all weaknesses. Until then, there is no recipe, so just keep playing until you've built up enough experience.

Again, as a rule of thumb, try to fix potential cuts, make sure every part of your group has enough eyespace or is solidly connected. If the inside area is very large, play somewhere in the center to make divide the potential invasion space in two fractions, each too small to do anything.