Specifying complex systems is a difficult task which cannot be done in one step. Step-by-step development processes have been studied using formal methods, based on refinement mechanisms. The refinement is a key feature for incrementally developing more and more detailed models, preserving correctness in each step. Our purpose is to instantiate this process when using UML/OCL notations. We illustrate it by some development steps of an access control system. At each step, decisions are formalized in terms of different UML notations, making evolve an initial model which expresses the fundamental properties of the system. We show that these properties are preserved by each development step.