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 X-KDE-PluginInfo-Name in the metadata.desktop file to match the new folder name. You will also want to edit the Name= in the metadata.desktop 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.json if it exists (or edit it as well).

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
kwriteconfig5 --file="$PWD/metadata.desktop" --group="Desktop Entry" --key="X-KDE-PluginInfo-Name" "com.github.zren.myanalogclock"
kwriteconfig5 --file="$PWD/metadata.desktop" --group="Desktop Entry" --key="Name" "My Analog Clock"
sed -i '/^Name\[/ d' ./metadata.desktop
sed -i '/^Comment\[/ d' ./metadata.desktop
rm ./metadata.json
plasmawindowed com.github.zren.myanalogclock