In installing flags (1" to 1 1/2" thick) on a 20' x 30' concrete patio
slab(which already has (away-from-the-house) pitch; what is best way to
maintain proper finished height of flags as they go down to keep that
pitch consistant?
A. String?
B. Set up side boards?
C. Use a scrap board ripped to the finished height depth as a guide?
D. None of the above.

Thanks

Are you doing the work or hiring out. I assume you mean to set in
concrete, if so set in a dry concrete, no water mix. Or what will you
set them in, sand? either way work them in to be the same level hight
and slope, its small enough. I would eyeball it since each piece is
different thickness, you dont want one high and one low.

D.
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
pitch.
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.