Measure twice, cut once
(carpentry, literally) One should double-check one's measurements for accuracy before cutting a piece of wood; otherwise it may be necessary to cut again,
wasting time and material. [quotations ▼]
1872, "Dressmaking," Hall's Journal of Health, vol. 19, no. 12, p. 280: Look at Carpenters! . . . In old times it was a proverb "Measure twice, and cut once."
(figuratively, by extension)
Plan and prepare in a careful, thorough manner before taking action.
Never cut twice!