This article describes a dissertation carried out by Mauricio Montoya Zuluaga and Cesar Calonge Saez. It aimed to design a methodological instrument for diagnosing phonetic-phonological and orthographical interferences of Spanish, as a native language, in English, as a foreign language. This methodological proposal is framed in contrastive linguistics and second language didactics. It is intended to be used by ESL teachers whose students’ native language is Spanish. The basis of this didactic strategy is absolutely theoretical (although it is intended to be put into practice) and it is supported on the contrastive analysis hypothesis whose main author was Robert Lado in his book Linguistics Across Cultures published in 1957, although it is enhanced by Rose Nash, Juan Manuel Serna, Juana Gil Fernandez, etc. According to the UPB Faculty of Education, the research methodology implemented in this work is called didactic material design.