generally speaking, first you want to remove the dome lights, visors, pillar covers, upper door trim and any A/C vents or plastic pins/clips that are up. With a sunroof, you'll also want to remove the rubber 'ring' that goes around on the inside.
After all those things have been removed, you'll want to open all the doors, and slowly try to begin working the headliner down. Be careful not to bend it, because you don't want any creases when you go to reinstall. Once you get on corner down low enough, you'll want to try and look inside there to see if there are any other clips, wires, etc. that may need to be removed to clear the headliner. You'll have to get one corner down first, and as you get the rest of it down, you'll need to rotate it carefully to get it out of one of the doors.
Then you'll need to be careful while carrying it - you don't want to fold it and get a crease. You also want to make sure you put it down on a clean surface. The padding behind it collects crap pretty easily and isn't exactly easy to clean.