There’s two ways to do this:
The online organizing way is to use a paid vpn to prevent local WiFi attacks and isp snooping, harden all devices and computers that will be used, only use encrypted communications and develop data security practices compatible with the possibility of being jammed up.
You gotta have real knowledge and awareness to pull this off as an individual, or a huge point of trust/failure in one person who does it for you.
If you do all that then the cops will just use metadata and inference to get warrants or just have someone infiltrate you or turn a trusted member.
The other way, the way the bolsheviks used, is to only organize in person and develop a no-trust framework for interacting with your community.
Then the cops will infiltrate or turn members but you have at least prepared for it and expect it.