Summarize:

Since slate tiles can vary slightly in size, it is important to do a dry run with your tiles so you know where to adjust the surface level and position of your tiles. Draw an "x" through the room with chalk. When the lines cross they will create a 90-degree angle on which you can check for even spacing. Use tile spacers during the dry run to ensure you have even spacing. If you do, you will want to reposition your layout, so that cut tiles of even width can be laid at either end. This will result in a symmetrical floor. Measure the tile to account for a one-eighth-inch (0.3-cm) grout space along the wall. You can cut slate tile with a diamond-blade wet saw, grinder or hacksaw with an abrasive blade. Place a line of chalk on them during your dry run, so that you can make this important change when you lay your mortar.
Sketch a layout for the color scheme of the slate. Measure lines through the width and length of the room. Lay out your tile to figure out the best use of color. Decide if you will need to cut tile at the edges of the room. Cut tiles to fit around the edges of the room. Mark tiles that require extra thinset on the back to create an even surface.