Inspiration Without Experience

Like many desires to write a song, I find my longing hits a wall when I realize how little experience I have in writing music. Lyrics? Gotcha covered, but the music? Not so much. I've filked for fun, parodied on improv, and even heard some strains in my mind (or hummed spontaneously) of something original I can only attribute to my grandfather trying to communicate with me through his own seldom-realized desire to write music (he wanted to be a composer, instead he was the head writer and contributing screenwriter for many a television show).

At present, I'm struggling with my desire to do a mash-up of "Gloria", "Gloria", and "Gloria". Quite specifically, I want to have three groups of singers, a few choral sopranos, a handful of women, and a handful of men, each with their own interwoven parts from Branigan, Morrison, and the Christmas traditional, "Angels We Have Heard on High".

Anyone with musical experience have any tips on how to proceed? I certainly have books (and a few classes, years in choir and musicals, etc.) to assist me in the most basic of technical aspects. But I could use a tip, a pointer, or even a resource or two. Blank sheet music and a pencil at the ready, just tell me how to press go!