Right-click on any file. Point to Actions... and Add template.... Then select "to user acount" or "system-wide". This opens a dialog to select an icon followed by a dialog to enter menu text.
From then on, you can right-click inside a folder, point to Create New > and select the menu item you created to make a new file based on your template.
Yes, it's pretty stupid that this is necessary, since there is a perfectly good Templates folder mechanism that works so much easier in those other desktops. I mean, how much easier does it get to be able to just drop a file in a folder and have it show up as a template in your context menu. But this IS KDE, after all.
This service menu uses a helper script written in bash. It was originally created for KDE 3. This updates it for version 4.