Just as electronic systems implement computation in terms of voltage (energy per unit charge), molecular systems compute in terms of chemical concentrations (molecules per unit volume). Prior work has established mechanisms for implementing logical and arithmetic functions including addition, multiplication, exponentiation, and logarithms with molecular reactions. In this paper, we present a general methodology for implementing synchronous sequential computation. We generate a four-phase clock signal through robust, sustained chemical oscillations. We implement memory elements by transferring concentrations between molecular types in alternating phases of the clock. We illustrate our design methodology with examples: a binary counter as well as a four-point, two-parallel FFT. We validate our designs through ODE simulations of mass-action chemical kinetics. We are exploring DNA-based computation via strand displacement as a possible experimental chassis.