Goadsporin (GS) is a member of ribosomally synthesized and post-translationally modified peptides (RiPPs), containing an N-terminal acetyl moiety, six azoles and two dehydroalanines in the peptidic main chain. Although the enzymes involved in GS biosynthesis have been defined, the principle of how the respective enzymes control the specific modifications remains elusive. Here we report a one-pot synthesis of GS using the enzymes reconstituted in the 'flexible' in vitro translation system, referred to as the FIT-GS system. This system allows us to readily prepare not only the precursor peptide from its synthetic DNA template but also 52 mutants, enabling us to dissect the modification determinants of GodA for each enzyme. The in vitro knowledge has also led us to successfully produce designer GS analogues in vivo. The methodology demonstrated in this work is also applicable to other RiPP biosynthesis, allowing us to rapidly investigate the principle of modification events with great ease.