Installing World Clock Deluxe is very easy.
Double-click the World Clock Deluxe 4.19.3.dmg file to mount the World Clock Deluxe 4.19.3 disk image.
Open the mounted volume and drag the World Clock Deluxe icon to your Applications folder.
If you are replacing an old version and macOS tells you that the operation can't be completed because you don't have permission to access some of the items, make sure the old version is not running and move it to the Trash. Then drag again version 4.19.3 to your Applications folder.