If you have to ask where to put masking tape, don't attempt a DIY paint job. Vinyl is much easier to apply, much more forgiving, and will give you a glossy paint-like finish that you won't be able to achieve without a professional paint booth and spray guns.
Edit: I didn't mean for that to sound derogatory, sorry if it did! It's hard to convey text in tone.
3M 1080 or Avery Automotive vinyl only. Don't settle for the cheap stuff that's harder to apply.
I would just follow the little gutter lines of the roof. I did a black roof on one of my S40s and loved it. Gave a nice contrast.