We conducted a study which examines mappings from a relatively unexplored perspective: how they are made. Twelve skilled NIME users designed a mapping from a T-Stick to a subtractive synthesizer, and were interviewed about their approach to mapping design. We present a thematic analysis of the interviews, with reference to data recordings captured while the designers worked. Our results suggest that the mapping design process is an iterative process that alternates between two working modes: diffuse exploration and directed experimentation.