- Cut off the old flares flush (better, a little recessed) from the body. This will expose both walls (inner and outer) of the wheel well housing, which makes up the flare. Most of the outer housing (the flare) will be gone.
- Insert the fiberglass flare into the gap between these two walls to make sure everything fits and no trimming of the fiberglass flare or remnants of the original flare is needed. You will be able to see the fiberglass flare from inside the body.
- Once the fit is OK, fasten the fiberglass flare. You can do this by pop riveting it to the two wheel well walls, but it is better (and recommended) to use a fiberglass gel adhesive. With this, you squirt/push the gel into the gap between the wheel well walls, slip in the fiberglass flare, clean up any residual mess, and let it dry in a well-ventilated place (usual fiberglass precautions).
Pop riveting is faster, but introduces new holes into the body, holds less well, and still needs to be sealed to keep water from getting back into your wheel well area and rotting it out. Using a gel adhesive introduces no holes, provides a more solid bond, and also acts as sealant.
Remember that these fiberglass flares are single thickness, unlike the original metal double-walled flares, and so can be visibly damaged by stones hitting the underside. Several layers of underbody coating or expanding foam applied to the underside of the flare (after it is on) will help protect it from stone damage.
The quarter panel to flare seam, if it has the original seal material, is probably dried out, hard, cracked, and leaking water. Clean it out (scraping, sand blasting, whatever you like), get rid of any rust you find (Extend or Extend-type rust converter is OK, but cold galvanizing works better and lasts much longer), and reseal it with a paintable body panel sealer. The new ones should stay flexible.
The materials you need for all this should be available from body shop suppliers or a quality auto parts store.