by guess and by gosh

by guess and by gosh

 and by guess and by golly
Rur. by estimating; without careful planning. Jane: Did you have a plan for putting up that toolshed? Tom: Nope, we just sort of did it by guess and by gosh. Grandma always made dresses by guess and by golly, without using a pattern, and they always turned out just fine.
See also: and, gosh, guess