Genetic code reprogramming is a method for the reassignment of arbitrary codons from proteinogenic amino acids to non-proteinogenic ones, and thus specific sequences of nonstandard peptides can be ribosomally expressed according to their mRNA templates. We here describe a protocol that facilitates the genetic code reprogramming using flexizymes integrated with a custom-made in vitro translation apparatus, referred to as the flexible in vitro translation (FIT) system. Flexizymes are flexible tRNA acylation ribozymes that enable the preparation of a diverse array of non-proteinogenic acyl-tRNAs. These acyl-tRNAs read vacant codons created in the FIT system, yielding the desired nonstandard peptides with diverse exotic structures, such as N -acyl groups, d -amino acids, N -methyl amino acids, and physiologically stable macrocyclic scaffolds. Facility of the protocol allows for a wide variety of applications in the synthesis of new classes of nonstandard peptides with biological functions.