Ayuh,.... Is this the 40'/ 50' yer talkin' 'bout in the knot thread,..??
To string line a swale, put a pin centered at the top, 'n another pin centered where ya want it to daylight,...
You'll need a pin at each turn, unless of course it's a straight line anyways,....
Pull a string line from pin to pin, makin' sure the elevation drops the entire way til it daylights,....
The 2x4 trick is a good one, as most rakes will clear under that point,....
Just dig/ rake, grade all but the 1/2' or 1' round the pins til it's done, pull the pins, 'n eyeball diggin' out the square foot ya left in place,....