All you need to do, after you've removed the loose tape and such, is sand the areas you're gonna apply joint compound to with, say ..., 150 grit sandpaper. You only need to 'roughen' it up. You DON'T need to sand down to the 'paper', or such. You only need to roughen it up enough for the compound to 'grab' onto the wall.
Be sure to lay mud in your joints first, before you put tape on, then IMMEDIATELY put your tape on, over which you apply your 1st coat of mud. Let it dry, and apply additional coatings while feathering out the mud more and more until you've reached the desired 'rough look'. Sand between coats and finish off w/220 grit sandpaper. (You may know all this but maybe not.)
WHen doing INSIDE corners (vs. outside corners), do ONE side at a time. After it dries, do the other side, and keep repeating the process until you've reached the desired finish. For OUTSIDE corners, you can do both 'sides' at the same time.