From
README.logging:
The easiest way for an end-user to define logs is to put the log in a separate XML file (usually under the user's home directory), then refer to it using the --config option, like
fgfs --config=log-config.xml
You can pass this --config option on a command line or via your favourite launcher. It's just setting up a branch of the property tree under "logging".
There is also a menu option, Debug | Logging, I think, that does the same thing but I couldn't get it to work last time I tried. YMMV.
Edit: I've updated the Wiki article, please revise it as necessary based on your success or otherwise.