How detailed should I describe my actions regarding code?
In general it is good to be complete, if you can do so without being overly pedantic. But the example you give (multiple data sets) seems like an especially important one to include. This is especially true as a single data set is seldom definitive (if the analysis relies on statistical data - sampling). Multiple, similar data sets gives general confirmation. 

If you say too little, you leave yourself open to questions about what you considered and didn't or whether your results are too sensitive to extraneous factors. 

Give enough information so that someone else is able to independently confirm your conclusion.