First, create a plugin class and implement either REIClientPlugin and REIServerPlugin, the difference between the two is outlined in Plugin Managers.

On Fabric, add the class as an entrypoint to rei_client or rei_server.

On Forge, annotate the class with @REIPlugin and insert the dists (CLIENT or DEDICATED_SERVER), into the annotation.

As a general rule, Annotate with CLIENT for client plugins, and annotate with bothCLIENT and DEDICATED_SERVER for server plugins.