pull the rocker cover off the car to get to the back side.
clean the back side with something like simple green and hot water.
then wipe down the area and about 4 inches past the damaged area with isopropyl alcohol.
sand with 80 grit sand paper and again 4 inches past the damaged area.
clean again with the isopropyl alcohol.
with masking tape . tape and hold the parts back together as it should be when not broken.
cut a piece or two of the drywall tape like shown in the pic's.
you want the tape to to fit the contour of the part with no wrinkles or bumps in the tape.
so cut the tape as need to do so.
put the tape to the side and using SEM 68422 plastic repair mix and then spread a tight coat over the back side of the damaged area.
stick the tape back on the damaged area and using a body filler spreader or putty knife push the drywall tape into the SEM plastic repair.
once the tape is pushed into the SEM spread another thin coat over the repair.
let it sit and don't screw around with the part once it dries then pull the masking tape off the outside of the part and clean up any dried ooze out you may have.
you can do this with a utility knife.
you can kind of see how it is in the pic's of the back side boats engine cover.
the engine cover is SMC plastic so the color of the repair product for that is black.