There may be other ways, but I use dbpoweramp (for Windows only).
get their main program, then get the flac plug in from the codecs page and install the main program first and then the flac extension program.
Then all you need to do is select the flac files, right-click on the selection, and "convert to" whatever, like MP3-Lame 320kbps CBR JS.
Unfortunately the current dbpoweramp has a 30-day limit on MP3 for copyright/patent reasons, I think. Previous versions had an unlimited (and technically illegal) MP3 encoder. If you can "find" dbpoweramp10, filename dMC-r10.exe, that version has no limits on MP3 encoding.