The only insane thing I've read is the use of mastic to install them. DONT!. Use a latex modified sanded thinset mortar with a 3/8" thinset mortar bed. Also you will need a 1 1/4" plywood or hardwood subfloor. On top of that you will require a ceramic tile backerboard like concrete backerboard or hardibacker board.
For a perfect cut I would take the slate to a marble and granite shop to have it cut into 12"x12" squares. It might cost 50 cents a foot or so but its money well spent. Slate usually starts off at about $2.00 a foot in price.
After the slate has been installed and before grouting seal your floor with a pre-grout sealer like this: