"Girding on armor" is metonymy for getting prepared for battle. "Tell Ben-Hadad, 'Do not boast as if you have already won a battle that you have not yet fought.'" (See: [[rc://en/ta/man/jit/figs-metonymy]])
This is a proverb, so you may want to make it as brief and easy to say as possible. Alternate translation: "Let him who puts his armor on not boast like him who takes it off" (See: [[rc://en/ta/man/jit/writing-proverbs]])