First, review posts with the #ChatControl hashtag, as there are lots of well-informed people sharing reliable information.
@quincy linked to a directory of the relevant contact points for each EU member state and requested that people write a (polite) letter, so if you have voting rights in the EU that's a good place to start.
As a leading voice of the German Pirate party and a member of EU parliament, Patrick Breyer (@echo_pbreyer) has actively opposed the proposed legislation throughout its lifetime.
He (and perhaps also his team?) have maintained a comprehensive, fact-checked summary of the proposal's status, with a practical "What can you do" section:
https://www.patrick-breyer.de/en/posts/chat-control/#WhatYouCanDo