Hi Roland,
Better follow the guidelines in the first part of the PDF manual, which you can find in Ukelele’s Help menu.
Two pieces of advice:
• Don’t install your custom keyboard layouts by pasting/dragging them into the Library folder.
• Best practice is to give your layouts version numbers or some kind of tag inside Ukelele, so that every new version has a different name.
That way, you can see which version you’re using (in the Flag menu) and also macOS won’t confuse them.
So how to install? Use Ukelele’s built-in Organiser:
Ukelele / File / Install / Show Organiser, click on "Set Folder…", navigate to the place where you saved your custom keyboard layout and drag it to one of the other columns to install it either for All Users or just for the Current User.
Then Ukelele will prompt you to log out your user and back in.
Keyboard layouts may appear without doing that, but certainly if you’re working with different versions, this is necessary to let macOS refresh its list of installed keyboard layouts.
Please post your progress :)