Is it possible to get small help with /etc/default/grub?

There is a thread where @dalto helped me a lot.
It is a long thread but this is where I started Btrfs Assistant 1.0 is coming, testers needed - #253 by dalto

Please be careful and read carefully.
This is where I was brand new with BTRFS, maybe I made something wrong, or posted something wrong.

But I am sure you can figure it out.
I will be glad to help though.
Feel free to ask. I will be glad to help or at least share my experience.

UPDATE:
It is mainly about installing BTRFS Assistant, snapper,… but you will need to issue a command to create the configuration. It is all there.

Important: I would suggest (I do this actually) I make a reply in that thread, write the commands I made for documentation and refference purpose, and after installing everything I suggest check with @dalto to make sure Grub is OK and if you need to do anything about Grub.If I were you I would do this.
If you are OK with Grub then it is fine.