Before laying any mortar, first arrange the stones over an existing 3-4 inch thick concrete slab in the desired pattern trying to keep consistent joints, most common are about 1/2 inch wide, and breaking up the joints so there are no continuous running lines. Stones may be hand-trimmed with a rock hammer or saw cut to achieve desired fit.
In one tub mix concrete using 1 part cement and 4 parts sand to create a firm mixture that you can ball up easily in your hand. You may also opt for a premix that you just add water to. In another tub, mix up some “concrete butter” which is a much softer mixture of plain cement, like that of soft butter, but not runny.
Remove a few of the stones, and lightly dampen the area of the concrete slab you’ll be working on. With a trowel lay a 1-inch or so thick layer of concrete and set the stones in it, lightly tapping them into place. The trowel handle works great for tapping. Check to ensure they are level and allow for a slight pitch for drainage. Try not to disturb the print of the fitted stones.
Making sure joints are clean, mix grout using 1 part cement and 1 part mason sand and prepare until stiff. Fill the joints with the grout and pack it in as tight as possible, trying to keep as much off the stone surface as you can. Strike the joints with a tool to allow for consistent, smooth joints.