First take the shiny color-matched covers off the bumper. It comes off very easy, just remove all the screws and tape/tie it up out of the way (the headlamp sprayer tube has some slack to it).
This is what you'll find underneath, just trim nice and tight along the top of the seam....
As you can see, this does not affect the factory fog lamps in any way. Normally you would see the washer fluid reservoir peeking through from this position. If it bothers you relocate it or paint it black.
Cut on the inside edge of the vertical seam. You'll want to go over this again when you get the panel off to make sure its nice and clean as this part will still be visible. I also trimmed the bottom/back edge inwards a little to round out the corner.