We're working on 2D tile sets for our maps, but we're not quite sure on the thought process for making a tile set. He understands what they are, how they're used, and he will be using the Tiled Map Editor though. He just doesn't know how to come up with how many different tiles will be needed, how to arrange them, etc. so we can create the maps in Tiled.
My suggestion was to first come up with a themes to group up all of our maps (cave, mountain, forest, town, indoors house, etc). Then, we would design a few maps in Tiled using a solid-colored block tiles to illustrate the layout of each map in Tiled. We could start visualizing from there, and come up with possible tiles needed to construct maps that'd actually be used in production.