Using a sod cutter, rakes, picks and shovels, remove all grass, plants and weeds.
Mark the area and then level, using rakes. Remove any excess soil.
Using recycled forms or inexpensive 2x4s, create the frame for the patio. Secure frame using 1' stakes.
Apply two inches of base rock inside of frame. Compact the area.
Since pieces of slate will be the top layer of the slab, the concrete doesn't have to look perfect underneath. Pour the concrete to the top of the forms and then float (smooth out) the concrete using a floating tool or wood 2x4s. Let the concrete harden over night or at least 12 hours. Check the concrete and make sure it's hard before continuing onto the next step. Using a hammer, carefully remove the forms used to frame the concrete.
Before securing the tiles to the concrete slab, make sure they will fit and decide on a pattern that works well with the design. Place the tiles and figure out where they will sit on the slab and which ones need to be cut.
Using a wet saw or tile saw, cut the tiles to size so they fit on the slab. Slate is extremely delicate, so buy more than you think you'll need. Some will break in the process.
The mortar, or thinset, will secure the slate to the concrete slab. Mix a small amount of the dry mortar with water to the right consistency, following instructions on the bag.
Slowly lay each piece in the chosen pattern and then place plastic "X" spacers in each corner of the tile, making sure each tile is equal distant apart and leaving spaces for the grout. The spacers should be 1/4" or 3/8". If you don't want to see the grout, jam pieces together.
Remove the plastic spacers. Mix the tile grout as specified on the package. With a sponge and bucket of water within reach, begin applying grout inside of the cracks. Wipe away the excess grout with a damp sponge. Let it dry. Give the patio overnight before walking on the surface so the tiles won't shift.