This paper presents the Centaur system through a tutorial describing the creation of an environment for a small language of mathematical expressions called Exp. With Centaur, the user may interactively generate programming language environments, including structured editors, debuggers, interpreters, and other tools. In this tutorial, all phases of language specification are covered : the design of the abstract and concrete syntax of Exp in Metal and Sdf, the pretty printing rules in Ppml, and the semantics of an interpreter in Typol. The tools generated by Centaur based on these specifications are enhanced by user interface built with Centaur graphic primitives.