I would go with test fit and make any mods, then paint, then do the final install.

 

Depends on what kind of lip you get. If you are getting some ebay knock off, JP vizage or a replica of a JDM authentic lip then yea test fit, trim, shave, sand, and fix all the defects to get it to fit. My JDM Auto couture lip didn't need all that. Just painted and put on. Voila! Done. My lip doesn't come up to the fog lights. And it just replaces the factor lip piece. I had it painted and put it on myself in 15mins. So the test fit is subjective to whatever type of lip you have.

 

Like everyone said...definitely test fit, modify (as needed), paint then install.