I have a large (14x20) wood louver with a frame that is going over a return vent. Should this be installed first and then have drywall put around it, or should I install the drywall first, then cut out a hole for the louver?
Put up drywall first - it is easier to get it parallel to close walls since you could install drywall 1-2% off. If you are not attaching this to a run and it is an open return I would also suggest trying to get at least one side next to a stud. Often the support holes for these are close to the opening so even with a good anchor these get flimsy.