Reading the about page is highly recommended.
Retentionizer expects csv data to have headers, with the first column named 't' representing time periods, starting at 0, counting up. Each additional column represents a cohort to model, as a survival percentage. The first cell (at t=0) for each cohort must be 1. Survival %s are expressed as decimal numbers. You can view a valid example csv file here , which contains the two cohorts from the original Fader & Hardie paper.