Make the readers [supervisor] understand the effort I put on data construction in an academically sound way
You are running into the age-old problem that, in research, you are reporting on results, not effort. Your paper (or thesis) is not a time sheet that needs to accurately reflect how much time went into which activity, but it should discuss your approach, results, and conclusions.

Hence, making your supervisor understand the effort [you] put on data construction is not a sound goal for a formal paper. I am not saying that your supervisor should not know how you spend your time, but this is an item for face-to-face discussion, not your thesis. Describe the data source, describe its properties (and, as a sidenote, I would not focus on the "it's free" aspect, but comment more on the quality of the data set), and describe the necessary transformations if they are interesting (instead of straight-forward but tedious, which most transformations seem to be).