Installation:

Drop all files in to the main Disco Elysium folder where the .exe is located.

Once in-game, press caps lock to toggle the HUD on or off.

Note that this will toggle ALL of the UI elements including menus, so you have to toggle it back on when using menus.


Configuration:

If you wish to change the key binding, simply open d3dx.ini and edit "Key" value in this section:

; HUD toggle for keyboard

[Key1]      
Key = VK_CAPITAL
x = 1
type = toggle

For A-Z and 0-9 on the number row, just use that single character. For everything else (including mouse buttons), use the virtual key name (with or without the VK_ prefix) or hex code from this article: 
http://msdn.microsoft.com/en-us/library/windows/desktop/dd375731(v=vs.85).aspx