First get the flac to wav converter at flac dot sourceforge dot net
Then get lame (you ought to have it already if you are encoding mp3s from CD).
The 'flac' program will convert the .flac file to a wav. Then use 'lame' to convert to mp3.
The defaults for both programs seemed to work just fine. So it was just a matter of doing:
flac -d *.flac
on the command line.