Removal of the bumper is not necessary. The diffuser is held clipped on by 6-7 push clips and yank it out...gently

 

Agreed. You should be able to go directly to step 4 in the instructions.. Just take your time and as Rocket states, gently.. If you can put it up in the air on a lift at eye level or rear jackstands, will make it a little easier..

 

You can also back it onto Rhino Ramps - makes getting underneath a lot easier, faster than jacking, and gives you lots of clearance. If you go underneath, and look up and to the rear, then you can see the clips these guys are talking about. They are loops in the diffuser, with body-coloured tabs from the bumper, holding all together. If any give you trouble, you can get a hand in from below and work them off. Also, some cars seem to have screws, some bigger push-clips on the outside bottom corners of the diffuser. Rest are the push-clip style.

Good luck!

 

Just did this a few weeks ago. No need to remove the bumper. Start with the fasteners on the bottom and then pull free of the clips at the top.