1. Don’t give your email address, or use a throwaway one when you join.
2. Pick a username that’s unrelated to any others you’ve used.
3. Use a VPN.
4. Don’t reveal personal details in posts & comments.

This just boils down to basic opsec. Be careful what you post/comment, continue to use a generic username, stuff like that.

in addition to what others have said, also have your browser fingerprint as fairly generic, and what is unique should ideally be randomised upon each start of your browser. There's nothing stopping a Lemmy instance from running clientside code that gathers your browser fingerprint, and if they are well-resourced enough to have access to fingerprint data from other sites, they could correlate it to de-anonymise you.

Just follow the basic social media rules and you'll be fine. Also don't trust anything that is clickable unless you hover over the link/copy it to some text editor.