If the plastic clip is part # 679885, my recollection is that there will be an opening in the track on the back of the trim. This opening (more of just a widening in the track opening) may be at the very end of the trim piece. You insert the oblong portion of the clip(s) into the opening and then position the clips over the holes in the body panel. Hold the trim piece in place (with a third and fourth hand) and push into place. A little bump with a large rubber mallet may help if they are reluctant to go in.
Where accessible in the fenders, you might want to seal the back of the opening for the retainer clips. The plastic clips might not rust; but, the paint in the opening will likely get chipped and start the inevitable rust process. A piece of this stuff rolled into a blob and shoved into the back of the opening can help slow the rust process
https://www.3mcanada.ca/3M/en_CA/p/d/v000074302/
I know that some people will paint the trim clip opening with a little POR 15. POR 15 is a very hard paint once it sets up and can retard the formation of rust in the hole opening. You do have to be careful that the POR 15 does not extend out under the trim on to the visible part of the panel. Once it sets up there is no removing it except be mechanical abrasion.