Okay, so laying out a map for a mosaic is just like planning any other cartography project. The only special need for my specific purpose here is that in the end of the planning phase I have the land selected. Here are two of about fifty or more options to achieve this result:
OPTION ONE: Scan In a Drawing.
The first image below is a hand sketch I scanned into my computer then opened into Photoshop. It is about 1000 pixels square, at 300 px/in resolution. (Remember, if your scan is not at this level when opened in PS you can go to Image>Image Size.). To get my selection, I used the Quick Selection Tool for the large areas and the Lasso Tool for everything else and to clean up the edges.
OPTION TWO: Draw PS Color Shapes
In the second image I created a blank PS page (File>New) about 1000 pixels square, at 300 px/in resolution. Then on a new layer above "background" I used a hard-edged brush in a random color to create my land masses. Then, simply ctrl-click the thumbnail of the color layer in the layers window (the little picture next to "Layer 1") and the land masses were selected.
Please note that whatever method you use, mosaic tile work by its nature is simplifying; thus unless you plan the image to be HUGE or the tiles themselves to by teeny-tiny, I do not recommend highly detailed coastline contours.
You can also see that I left a wide border around the edge of the document; this is because I plan to have a fancy-schmancy border around the final map. You can, if you choose, take your map all the way to the edge.
NEXT: Beginning to tile!