First of all, make sure the tiles you are laying are calibrated. You will find that the cheap slate tiles are more than likely uncalibrated. In other words, the dimensions of the tile will vary a mil or 2, and the thickness will vary even more. This is because the tile is cut out of slate into blocks and then pulled apart at the seam to form your tile.
If they are uncalibrated, then you don't use spacers but use your 'eye' spacers instead, to lay them as best you can. You must also explain this to the customer the fact that the grout lines will not be identical throughout the floor. The last thing you want is the customer to think that you have done a rubbish job, when it wasn't your fault.
When it comes to laying, use your adhesive as you would do with any floor, making sure you try not to get any adhesive on the surface of the tiles. If you do, then just wipe them before moving on.
Once the floor is laid, then leave to dry and seal using a suitable sealer. Either use a mini roller or a sponge if the tile surface is very jagged. Some people like to seal before they lay the tiles - up to you, but doing it when the floor is down is a lot quicker.
Once the seal has dryed, grout as normal - using a grout that matches the adhesive (Flexi / non flexi etc).
Oh, and trying to use your Rubi cutter to cut these tiles is a big no-no. Its wet cut all the way with these puppies, so spend some time setting out to ensure you have no smidgen cuts as best possible and keep the water trough in your cutter full and fresh
