Use it exactly as it is. Don't change the address or download the file: the chat updates itself from there.
You only paste it once. Colours, messages and everything else change from the dashboard and reach your site within about a minute.
Add the code with Code Injection
In your Ghost admin, open Settings.
Go to Advanced › Code Injection.
Paste your install code into Site Footer. Code there goes just before </body> on every page.
Save.
Readers and members see the same chat, and you answer them all from one inbox.
The chat on some pages only
Each post and page has its own Code Injection in its settings, if you only want the chat on a few. Or keep it in Site Footer and list the pages to skip under Chat Widget › Behavior › Pages without the chat.
If the chat doesn't show on Ghost
The code is in Site Header. Move it to Site Footer.
Your theme doesn't print the footer code. Ghost themes include it with {{ghost_foot}} just before </body>. Almost every theme has it; if a custom theme doesn't, add it to its default.hbs.
Check that it works
Open your site in a private (incognito) window, so you see it the way a visitor does.
Look for the chat button in the bottom corner of the page, then send yourself a test message.
In the dashboard, the message appears in your Inbox as it arrives. Reply, and watch the answer reach the website.
You can also choose check the installation on the Install tab of Administration › Chat Widget: we load your home page and look for your code. No button? Go through when the chat doesn't show.
Set up the rest
The chat is on your Ghost site. A few minutes in the dashboard makes it yours:
