The Discrete Empirical Interpolation Method (DEIM) is a technique for model reduction of non-linear dynamical systems. It is based upon a modification to proper orthogonal decomposition which is designed to reduce the computational complexity for evaluating the reduced order nonlinear term. The DEIM approach is based upon an interpolatory projection and only requires evaluation of a few selected components of the original nonlinear term. Thus, implementation of the reduced order nonlinear term requires a new code to be derived from the original code for evaluating the nonlinearity. This work describes a methodology for automatically deriving a code for the reduced order nonlinearity directly from the original nonlinear code. The methodology is derived from standard techniques of automatic differentiation. This capability removes the possibly tedious and error prone task of producing such a code by hand and hence can facilitate the use of DEIM by non-experts.