It is very straightforward, I'll explain anyway.
(testet in Catalina)
Download the newest version for macOS from releases.
Open the .dmg, start "Keyboard installer" from there. Pull the file to the text on the bottom, click on "User". ("For all" is apparently deprecated)
!! WITH "USER", THE LAYOUT IS NOT AVAILABLE ON THE LOGIN SCREEN !!
- System Preferences -> Keyboard -> Input Sources ->
+
- The layout can be found in "others" (all the way down). You can also type "nizo" in the search bar.
- Select layout, press "Add"
- If the first checkbox in the Settings menu is ticked, the status bar has an icon for switching the layout. Select my layout here.