I used cable ties originally, just to see if I liked it, then, when I decided I did like it, I moved to the self tappers, although the cable ties held it perfectly well.
Get down under the back of your car and take a look, it's pretty self explanatory once you see what's involved.
The mid section of the bumper is held to a support/impact beam by two tabs. These tabs attach to two white plastic brackets. Unscrew your bumper from the brackets then either remove the brackets or, I simply turned them anti clockwise 180 degrees so they're outta the way. You can then cable tie the tabs on the bumper to the upturned brackets, or screw the tabs to the support/impact beam.
Others have screwed through the bottom of the bumper, but I didn't fancy this. So I'm only using what Lord Almighty Nissan provided (the two tabs) and haven't really got any sag.
Simples.