[britta]

Keep learning, keep reading, keep building empathy and understanding, keep piecing together an understanding of what people mean when they say privilege and why they think it's important, learn more about the history of feminism and of the different approaches to feminism, skim some articles about intersectionality; over time you'll notice more subtle forms of sexism, develop a better vocabulary for identifying it, and develop a better instinct for what you should do.

Examples off the top of my head: sometimes it may be as simple as noticing that people keep unconsciously interrupting a female coworker for no real reason, so you casually and non-condescendingly form a pause so she can finish talking - or maybe you're helping pick speakers for an event and you've thought of a person who happens to be a woman who would be great to round out the day, but she hasn't submitted a proposal yet or whatever, so you email her to ask if she's interested. And of course you also keep a friendly non-condescending eye out for men who similarly may be getting unfairly ignored or underestimating their own skills, but that kind of subtle social support often already comes naturally for people dealing with their own gender, and it's also somewhat less common for men to be randomly assumed to not know what they're talking about, etc.

In other words, being an active feminist is just generally being a decent human being, to women and to men, but also including a well-informed eye toward the biases left by generations of discrimination.