There is a clip on AE86 club vol. ? I think 8?
Have just searched youtube, but couldn't find it.
Have a look on Club4ag, there is a thread and video links on there.

I have mine attached with 4 90deg steel brackets from the lip mounting points, to the bumper, then have self tappers through the the bumper into the corner female attaching holes.
No need to trim the bumper or lip, the only thing needed is the 2 holes drilled on the corners of the bumper for self tappers.
Also, the brackets should align with holes already in the bumper.