Plasma Widget Tutorial

The KDE wiki has a Getting Started and Hello World tutorial which you can read as well.

Default Widgets

Learn By Example

You can learn by example by reading the default widgets located at:
/usr/share/plasma/plasmoids/

You can fork an existing widget by copying the widget to where the downloaded widgets are installed to:
~/.local/share/plasma/plasmoids/

To properly fork the widget, rename the new folder, then edit the Id in the metadata.json file to match the new folder name. You will also want to edit the Name in the metadata.json file as well so you can tell it apart from the widget you forked it from. You should delete all the translated Name[fr] lines as well. Delete the metadata.desktop if it exists.

Finally, run plasmawindowed to quickly test the newly forked widget.

cd /usr/share/plasma/plasmoids/
mkdir -p ~/.local/share/plasma/plasmoids
cp -r /usr/share/plasma/plasmoids/org.kde.plasma.analogclock/ ~/.local/share/plasma/plasmoids/
cd ~/.local/share/plasma/plasmoids/
mv ./org.kde.plasma.analogclock ./com.github.zren.myanalogclock
cd ./com.github.zren.myanalogclock
sed -i 's/Analog Clock/My Analog Clock/' ./metadata.json
sed -i 's/org.kde.plasma.analogclock/com.github.zren.myanalogclock/' ./metadata.json
sed -i '/^Name\[/ d' ./metadata.json
sed -i '/^Comment\[/ d' ./metadata.json
rm ./metadata.desktop
plasmawindowed com.github.zren.myanalogclock