If you are interested in EA research/an EA research job, I would recommend just reading EA research on this forum and on the websites of EA research organisations. Much of this research doesn't involve any research method beyond general desk/secondary research, i.e. reading relevant literature and synthesising it.
In the cases where you see EA research relies on some specific technical methodology, such as stats, cost-effectiveness modelling, surveys etc., I would just recommend googling the specific method and finding resources that way. In general, I think there are too many different methods and approaches even within these categories, for it to be too helpful to link to a general introduction to stats (although here's one, for example, since depending on what you want to do, a lot won't be relevant.