An addendum to the tut: One thing I forgot that can neaten things up down the road is this.

After Step 7 (when you delete all the land that isn't inside the line box), you can take your Eraser tool and erase the strip of land/white area that runs all the way around UNDERNEATH the line box. That way, when you set up your concentric rings in the next step, you won't have rings around the outside of the map. Just be careful to ONLY erase the parts that are over the water areas, not the landmasses themselves.

Sorry about not including that originally; that's what happens when you rush through something. Well, that and typos.