How do you maintain a round circle if your building a wishing well made of native stone?

Question

The rock well will be about 4 feet in diameter and about 4 feet tall.

Answer 1

The first question is what is the native stone? If it's flat/ie sandstone not a problem if it's round or odd shaped, I guess you better have a good eye and keep the level and tape measure handy to keep it the same width in diameter and level all the way up.
I think when measuring the diameter measure it on the inside to keep it more accurate as opposed to the outside.
This would be the easiest way to maintain your circumference by the continued measuring of the diameter and checking it for level
As far as making the beginning circle RScott is correct.

Answer 2

Drive a stake in the ground and attach a two foot string to the post. RScott