Various investigations agree that the variable concept has a multifaceted character, in algebraic problems it is presented with different definitions, references and symbols, so that students show difficulty in interpreting and putting into play its various uses. In this article we present the design and implementation of a study process on the variable, which allows students to understand its uses. For the development of the research we used the stages of Design-Based Research (DBR) and the notions of didactic trajectory and didactic suitability introduced by the Ontosemiotic Approach to mathematical knowledge and instruction (EOS). The development of three sessions with a group of Mexican high school students is analyzed. The retrospective analysis between the design and implementation stages allowed us to identify the acquisition of algebraic skills. These results could guide the design of didactic experiences.