Create plugin to store user group as cookie value

Hi,

I've setup a dev enviroment and set IPB to dev mode, and have been trying to learn to create a plugin which sets a cookie with the users group ID.

I'm really struggling with this and there appears to be no documentation: https://invisioncommunity.com/4guides/developing-plugins-and-applications/creating-plugins/

When I create a new plugin and select theme hook, it take me to a split screen where I can't edit anything and I'm really not sure what I'm supposed to do.

Any help would be most appreciated.

Thanks