Well the first step is to be proactive and work on building the new community. Use the tools you have at your disposal and your knowledge in the topics you love to add content! Not a great content finder/poster (no shame, I've never been either), then at least engage with what's there. Discuss with the people (they aren't bots here) that are also interested. Foster an environment you want to spend more time in and you'll inevitably do that!
Second, and I'm honestly surprised more people aren't talking about this aspect but for now - POACH. Reddit is not dying or going away the way the confirmation-biased rexxitors would have you believe based on all the death of Reddit style posts (hopefully one day but it's not today, and it's not in the short-to-near term). That means those content communities and circles are still alive, if slightly smaller (hopefully) and people (and definitely Reddits bot army) are likely still keeping them populated. Steal it! Bring that shit over here! Reddit is a link sharing sites primarily, and so is this! Take that link, ctrl-c, ctrl-v baby!
Finally, spread the word! Those that haven't rexxited haven't done so because either they haven't heard about Lemmy/kbin/fediverse, it's too confusing, or they haven't seen an enticing enough reason to. None of those things will change without engaging with them.
Cheers!