Some relevant points:

• Wet clay shinks about 13% so your model needs to be larger than the finished, fired tile. You may need to run a shrinkage test by rolling out some clay and tracing a ruler's gradient, then dry and fire the tile. Comparing the ruler to the marks on the fired clay should give you a good idea how much that particular clay body shrinks.
• For the tile to slip free from the mold there can be no undercuts. Patterns that require undercuts either must be pice molds (beyond the scope of this tutorial) or the undercuts need to me made after the tile is free of the mold (a two step process).
• Slip is required for ALL fired additions. If you do not intend to fire your model (which could even be made of oil-based clay (Plasteline) if never fired) you MUST use slip. If you do not intend to fire your model then water is enough "glue."

Cut your tile "blank" to wet size. I am, for example, currently using earthenware clay from Continental Clay in Minneapolis. I get a perfect 6" tile from a blank 6 3/8". The wet tiles will come out of the mold at about the same size as your model so you should not need to allow for in-the-mold shrinkage. Cut the sides at about a 5 degree angle (the sides slope out slightly).

Roll out more clay to the thickness of your build-up layers. Cut the shapes you want and apply with slip. Bag in plastic.

The moisture in the slip will seep into the tile and after a while will equalize. When this has happened you can go in with wooden sculpting tools and smooth and shape. With a little more time (and perhaps slight drying) you can use steel tools to carve finner detail.