A simple way of doing this on Windows (If you prefer not to use a script) would be to use FooBar2000.
Install FooBar2000, then drag your your Music folder containing the albums with flac files into FooBar's main window, this will add them to the queue. If you have a large collection, this could take some time.
Make sure all of them are selected (Ctrl+a), right click, go to convert, and click the three dots.
You're now presented with the conversion menu. You can set the output format to MP3 and edit the settings to your liking. The destination is something you will want to change.
Set the "Output Path" to specify where you want the MP3 files to go, probably another folder for your MP3 output. You can then set the name format as follows:
%artist% - %album%\%track% - %title%
Of course you can modify it to your liking.
Then simply press back, change any other settings you may be interested in and click "convert". You can also save these settings to use them again later.
Note you may need to download lame.exe separately for the conversion to MP3 to work.