Some relevant points:
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.
Slip is a very wet mix of the same clay body used to "glue" clay together. What I do is roll out clay very thin and let it dry (do not force dry with heat). When dry break the clay into a sealable container and add water. Letting the clay sit in the water you will see it slowly break down. When this is complete (you may have to add water) the clay will be easy to stir.
You can do it the hard (but fast) way by breaking wet clay into small pieces and adding water slowly while stirring vigorously. I used to do it this way. Now I plan better.