The "fl" spelling is an attempt at approximating the sound of Welsh "ll," which is simply a voiceless "l." The "l" after an "f" (or any other voiceless consonant) in English is normally voiceless (though you can make it voiced if you try), so all you have to do is leave off the "f" and you've got the sound! As I point out in my similar note to "Lloyd," it's not hard, is it!