How should I get started?

This isn't prescriptive, but to start, you can try using userscript plugins like "greasemonkey" and "tampermonkey" in order to inject your code into a running corru session.

A more advanced way, easiest if you already have some terminal experience (though you might want to try this to get your feet wet in the more advanced development techniques), is to:

Download a corru html page you want to iterate on, putting it in your project folder e.g. `cd myproject; wget https://b.corru.observer/local/ocean/embassy/ -O index.html`
Install the `http-server` npm package, and run it in your project folder, with a proxy to the main corru server, so that you can grab all the assets you're not overriding from the main corru project, like so: `http-server -P https://b.corru.observer/`
Download the js/corru.js file (at js/corru.js in your project folder) to make a specific override - in particular, at line 2051, you want to add the following extra argument to the `fetch` on that line: `, { mode: "cors" }` (so the fetch will now look like: `fetch("https://state.corru.network/", {mode: "cors"})`. This will allow you to perform the connection to the gmss system, allowing loading to complete.
If you want to modify any other asset, you can download a local copy at the path it would normally be, and modify it, to see the modified version on your own machine

This has the benefits of being able to load entire HTML documents with the context of corru.observer, which allows you to test and iterate on extreme changes to the corru.observer environment.