Use a level to note the current pitch of the concrete patio slab.
Put down a 1 inch layer of sand.
Start at the wall-sides.
If the flagstone is 1 1/2 inches, work it down to a half inch.
If the flagstone is 1 inch, place it on top of the sand.
Use the level as you go along. Err on the side of having more of a
You should end up with the stones on the grass side of the patio having
1/2 inch of sand below the 1 1/2 inch stones. Your erring on the side
of having more of a pitch might make this amount 0 inches.
Mortar between all the stones and at the edges.