From the official wiki

Your very first mod

This is a beginner-friendly walkthrough based on the official Stellaris Wiki modding tutorial. It focuses on one concrete goal: ship a tiny mod you can brag about—a custom civic with a name only you would pick, proper text in-game, maybe an icon and a cheeky event—so you are not just reading rules, you are assembling something playable. Unless stated otherwise, the steps assume Windows. It follows the wiki’s Modding tutorial in order, but frames each step toward that payoff. Use Next or the sidebar; your place is saved in the URL (?step=). Written with patch 4.3.3 in mind; always match supported_version to your launcher.