A .m3u playlist in it’s simplest form is simply a text file with filenames..
I occasionally make playlists at a DOS prompt with:
dir /s/b *.mp3 > playlist.m3u
(the /b shows a basic listing, with no date/size information)
(the /s will go through subdirectories and find files in all of those. Also, this is required in order to show full paths, which are always good for .m3u files..)
Replace *.mp3 with *.flac or *.* as you wish, and further edit with a text editor!!!