The sentence "If you don't want to use the key for mouse toggling, simply remap it to another key." is a little bit confusing to beginners.

What does "simply remap" mean?

I for example have F12 set to togglepaste and would like to have F11 set to togglemouse but so fall all my attempts have failed.

What is the correct form to remap it in .vimrc?

Your guide says

If you don't want to use the <F12> key for mouse toggling, simply remap it to another key. It autodetects whether it has been remapped and won't register the <F12> key if so.

I'm confused, I'm not sure how to "remap" a key (when I look this up all I find are things like noremap, which is a recursion free key mapping definition). I took a look at the code and it looks like if I could map a key to your s:ToggleMouse() function (before loading your plugin) then that would work and <F12> would not be mapped. The problem is I don't know how to call that function because it looks like s is replaced with a autogenerated name by vim to avoid naming conflicts.

How are you supposed to do this?

I'm a total vim noob. Just tried to clone the repo into my bundle folder, but F12 doesn't seem to have any effect.
I also tried to rename your plugin folder to ftplugin, again with no luck...

Is there really no second step?? :)