Clone the repository and run plasma package manager to install:
$ git clone https://github.com/baranga/1und1-data-usage-kde.git
$ cd 1und1-data-usage-kde
$ plasmapkg2 -i plasmoid
Pull and rerun plasma package manager to update:
$ cd 1und1-data-usage-kde
$ git pull
$ plasmapkg2 -u plasmoid
Install/update and then run plasmoidviewer
or plasmawindowed
to test:
$ plasmoidviewer -a de.baranga.1und1-data-usage-widget
$ plasmawindowed de.baranga.1und1-data-usage-widget