To start translating your website and access the Overlay Editor you don't need to install the code snippet yet. You may start working on your translation project immediatelly after creating it.
However, to make your translation live, you'll need to install the code snippet in the HEAD section of your website's HTML code.
This will show a language selector on your website after the translations have been published.
Every website requires a unique code snippet. Even if you have multiple different websites under a same root domain (e.g. www.yourwebsite.com, api.yourwebsite.com, docs.yourwebsite.com), you will have to create a separate project for every website and therefore install the corresponding snippet of that project into the website.
It is important not to install more than one code snippet on your website. By creating a website translation project through Text United web portal, you will get one code snippet for your website (regardless how many languages you are translating to) and you must install only that snippet. Later on, if for whatever reason, you had to create a new website translation project for the same website, you will have to remove the old snippet and install only the latest one.