A guy did a youtube vid about it a long while back. I installed it on the sides only and made it longer, almost top to bottom coverage. Keeps the water out of those side gaps. I kept the floor hinge area open for water drainage when it's washed or opened in rain. No aftermarket tailgate hole cover either, I keep it vented for moisture as designed.
Clean the area, then wipe with isopropyl alcohol. Apply weather stripping on thesides & bottom of the bedside tailgate opening. Keeps out alot of water and debris.