Read No Shortcuts [1]. But in summary, determine the group of people you want to organize (a bounded group, not "everyone"), reach out to people (face to face conversations), identify community leaders who are on board, try doing a small bit of activism, and see what percent of your target group showed up. Identify leaders who got people to show up, and look for different leaders where they didn't work. Repeat, using any success as incentive to get more people interested in participating next time.
[1] https://global.oup.com/academic/product/no-shortcuts-9780190...