You can start with reading about methods. Or you might choose to take a workshop or enroll in a class or two. Finding collaborators with the expertise you need might be an option.
In addition to reading books about methods, don't overlook the benefit of reading published studies. Not only do you get to read interesting findings, you will also learn about the process of research and you'll see examples of how to present data. Papers will generally have sections describing methods (e.g., if authors used a survey, how did they select a sample? how did they keep track of participants? how many responses did they get? how did they code answers?). They might also have an appendix with more details.
Graphic: Newton's Revolving Disc for Re-Composing Colours into White Light, from The Half Hour Library of Travel, Nature and Science for Young Readers (1896), available in British Library's photostream on Flickr
Experienced empirical researchers offer workshops aimed at legal scholars. For example, Northwestern University's Pritzker School of Law is offering a Workshop on Research Design for Causal Inference in summer 2024.
Sources for further workshop announcements include:
Graduate students (and sometimes undergrads) in the social sciences are required to take classes in the research methods of their fields. A partial list at the UW includes;
Anthropology
Communications
Political Science
Psychology
Sociology
Foster School of Business
Evans School of Public Policy and Governance
School of Public Health
School of Social Work
Graphic: Poster by Sam Braun, Federal Art Project, 1937. "The World Wants New Knowledge, New Skills--Enroll: Federal Adult Schools." Library of Congress Prints and Photographs Division.