How do I make a window transparent in KDE?

1) First Go to System Settings → Window Behavior

2) Open Window Behavior, Select Window Actions by again selecting Window Behavior

3) In Inner Window, Titlebar & Frame section, Use the Mouse Wheel action to Change opacity

Here is a transparent window

1. Click on window actions (top left corner) → More Actions → Special Window Settings

   

2. Go to Appearance & Fixes tab: force any transparency you want for active / inactive state.

   

You can also manage all created rules in single place as with compiz through:

System Settings → Window Behavior → Window Rules

System Settings > Shortcuts & Gestures > Global Shortcuts, then select "KWin" from the drop down. Find the options "Decrease opacity of Active window by 5%" and "Increase opacity of Active Window by 5%".

Additional info: https://forum.kde.org/viewtopic.php?f=212&t=98612