To create an .m3u/.m3u8 file in Winamp go to the Playlist window
and click List Opts->Save Playlist and then simply save the file.
In iTunes, you first want to do an "Export Song List..."
You can right-click on "Music"...
...or you can right-click on a playlist to get this menu option.
Then be sure to save as a .txt file and not
as an .xml file.