Wonderful stuff--Mason's line. It comes in colors, is very thin, stretches till it's twangs like a violin string--tensile strength of something like 175lbs before it snaps.

Set the string 3 1/2" inches above where you want the soil--the exact width of a 2x4--and level that. You can use a water lever, a laser level or a string level. So your string is 1 2x4 thickness above your desired level. Then just use the 2x4 to check.

Run a few strings so you always have a place to check.

First saw the Red Clay Ramblers at the Station, in Carrboro, in 1978. I was a carpenter down there then, so that red clay was quite the nemeis--I remember it yanked the sole off my boot! Rain made it splatter and a job site could be spattered up as high as 4 feet...and that red clay is about as hard to get out as a waterproof Sharpie!