Filter configuration file may be editted directly using any text editor. But it may be more covenient to use Foxy web-based Filter Editor.
Point your browser to http://foxy/filters (actual host name may be different, depending on the value of the pseudo_domain parameter defined in the main configuration file.)
Click on a filter you want to edit. Functionality of the buttons on the bottom of the form should be self-explanatory.
Click Clone to make a copy of the current filter.
Click Raw to switch to raw editting mode. In raw mode filter configuration looks exactly like it looks in the filter configuration file.
Click Form to switch back to form mode. In form mode you don't have to know the exact parameter names, you just enter values. List parameters (e.g. include) are editted in text boxes, one value per line.
It is not recommended to edit filters in raw mode. Use it when you want to find out which parameter exactly a form element represents. Filter documentation does not mention web-based editor, all parameter are described as they are stored in filters.cfg and raw editting mode.
The buttons on the top of the filter form are used to change filter order, e.g. << moves filter to the beginning of the list.
Don't forget to click Apply before moving to another filter.