I might consider doing this for you if I have the time this week (never made a video tutorial before, however, but I have written an online highscore system).
However, before you bother offering cash I'm sure we could always just help you out in the programming section.
You said you already have a server so have you written any code where your server talks to your database? If so, all you need to do is literally add the step of Godot talking to your server and then let the server pass it on to the database.
If you know PHP (and MySQL, of course) then you're good to go.
Again, this is working off the assumption you have designed even a very simplistic website on your server but the steps are like this:
Make a .php page on your server that takes in POST requests, parses the data, and talks to your database. Assume anything echoed from this page will be received by Godot (including, for example, a JSON response). If you are using something like PDO (a standard class in PHP) for your MySQL requests then most SQL injection is already taken care of.
In your Godot project simply use an HTTPRequest to send a POST message to that page on your server. The server then responds by echoing whatever it needs to which is then received by Godot. Godot has a built-in JSON parser that can go from Dictionary to String and back so that makes that portion trivial.
That's about it. For something like highscores you'll want some extra security like nonce checks to help prevent cheating (something easily implemented), a security hash (again, something easily implemented), and, if you plan on hosting your Godot game online somewhere other than your server (such is Newgrounds, Miniclip, Itch.IO, or a number of others) you'll need to have some pre-flight check code in your PHP (again, a couple lines of code).
I realize all this waffling above is uninvited so if you still want a video I'll think about making a simple walkthrough for this but otherwise I'd be glad to answer questions if you ask them in the Programming subforum.
NOTE: I am not an expert in this area, but I have over 15 years experience towards programming in general and have learned all the above just researching on my own.