Liquid droplets of aggregation-prone proteins, which become hydrogels or form amyloid fibrils, are a potential target for drug discovery. In this study, we proposed an experiment-guided protocol for characterizing the design grammar of peptides that can regulate droplet formation and aggregation. The protocol essentially involves investigation of 19 amino acid additives and polymerization of the identified amino acids. As a proof of concept, we applied this protocol to fused in sarcoma (FUS) and succeeded the peptide binder design and aggregation regulation. The developed protocol could be used for the primary design of peptides controlling liquid droplets and aggregates of proteins. Also, I may introduce the development of rational peptide binder design method.