Extract from:
http://oldbugs.calibre-ebook.com/wiki/RecipeTips
NOTE: you are strongly advised NOT to edit the built-in recipes directly from the recipes folder!
The second method is the recommended one and here is how you go about it.
In the main window of calibre click the little arrow next to the "Fetch News" button and then click on "Add a custom news source".
A new window opens up and on the bottom left corner click on "Customize builtin recipe".
Now a little window opens up with a drop down box where you can pick the recipe of the news scource you wish to customize.
Once you have chosen a particular news source it should appear in the list on the left column of the window.
Select it in the left column and the recipe will appear on the right column of the window.