In the context of the executable specification language OBJ3 an order-sorted completion procedure is implemented, providing automatically convergent specifications from user-given ones. This feature is of first importance to ensure unambiguity and termination of the rewriting execution process. We describe here how we specified a modular completion design in terms of inference rules and control language, using OBJ3 itself. On another hand, the specific problems encountered to integrate a completion process in an already reduction-oriented environment are pointed out.