Try this one and see what you think. I'd be very interested to hear responses. The best approach I ever heard was by Earl Robinson, in or about 1956, who said something like this: (1) Folk music is music which has been smoothed over time by passing from one individual to another, each making a small contribution, until the music is perfected in ways no individual composer could achieve. (2) In order for the folk process to work, the people involved must understand the music they perform. They must either be raised in a musical tradition, or must pay careful attention to it and follow it. (3) Robinson, who composed the "Ballad for Americans," "Sandhog" and "Joe Hill," among other things (and was very proud of it) did not call himself a composer of folk music. He said he had heard changed versions of "Joe Hill," and thought it would evolve into a folk song. (4) Robinson thought that classical music was great only to the extent it derived from folk roots. He argued, this is why Bartok is a greater composer than Schoenberg. (I'm not sure whether I agree, but it is an interesting point.) (5) These were political points Robinson was making, as he considered folk music to be the property of the Left.
In the same vein, I heard Bess Hawes say that she had taught a course in singing folk songs. One week, for an exercise, she had all her students choose a well-known singer and imitate him/her as closely as they could. When the class met, no one could tell who it was that anyone was trying to imitate. But ALL the students sang better than they ever had before. In other words, they improved because they had to think about what they were doing.
That being said, I suggest the following, and ask for comment: (1) Folk songs are SIMPLE songs. Because folk songs do not have a lot of complicated instrumentation, chord progressions, etc. they derive their force from very small things, like small variations in rhythm and vocal inflection. The best folk recording I have ever heard is "Alabama Bound" with Leadbelly and the Golden Gate Quartet. All either a capella or with a single guitar line, but the rhythm and phrasing are perfect. (2) The best folk music is performed by people who are raised in a tradition and stick to it. Good folk music requires that the performer at least treat the tradition with respect. There are trained opera singers who patronizingly include a few simple folk songs in their repertoire. No matter how magnificent their voices are, they rarely sound good. (3) The best folk music is moving because of a nuanced combination of words, tune, rhythm, vocal inflection, and instrumentation (if any). A folk singer raised within a tradition may understand these things without needing to think about them. An outsider who wants to sing the songs should think about them very carefully. That is why Leadbelly is better than Odetta, and Odetta is superior to Judy Collins. That is also why Prof. Child was wrong when he said the words of a ballad are more important than the tune. He was wrong because it makes no sense AT ALL to think about the words in isolation from the tune, or even in isolation from the singer's accent and phrasing. (4) Songs composed by singer-songwriters may be good, but they are not folk music. And the more complex the composition gets, the farther it probably is from anything that could be called folk music. (4) The idea of creating "fusion" music, or bringing all the traditions together to create something to unite mankind, is wrongheaded. Homogenized music is like Kraft homogenized cheese. The idea of "liberating" music from rules is wrongheaded. Traditional music is good because it is traditional. Traditions are local and have rules. (5) Although folk songs are associated with the Left, Songs of Protest are not necessarily folk songs. And a bad song does not become a good one because the sentiment is good, or politically correct.JWM