In this article, we share the design and implementation of the preparatory phase of a didactical experiment with teachers of mathematics, using informatics technologies, in classrooms intended for young teenagers from 12 to 17 years old from the province of Buenos Aires, Argentina. We based this design on the contributions of different theoretical frameworks (MTSK, Mathematical Working Space, Collaborative Research, Teaching experiment methodology) and analysed the data obtained from the administration of an instrument created ad hoc, to generate information that will allow us to build the didactical experiment. We conclude that the design of the experiment requires a joint reflection between the groups of teachers and researchers, to make the teacher?s specialised knowledge explicit, guarantee its performance and goal achieving