Advice for general HRE emperor stuff? Trying to get "Everything's Coming Up Mulhouse" achievement

Like the title. Never done a game in the HRE before. Right now I just became emperor, but the Protestant Reformation is in the middle of happening so there's the big authority malus from heretic princes. Right now I have one(1) province Protestant, but I was thinking of flipping to Prot. Guess I'll go whichever way has the majority.

Another thing I'm trying to wrap my head around is how to make more free cities.

Also there's some lands that Burgundy owns that are in the Empire but Burgundy isn't, but I was planning on decing on France to force them to liberate the HRE land.