Tā bǎ shìqing de láilóngqùmài yī èr sān sì de jiěshì qīngchǔ le.
He explained the whole matter clearly, step by step.
Tips
usage
一二三四 is often used rhythmically for drill commands, exercise counts, or figuratively to mean 'systematically' or 'step by step' when listing things in order.