I don’t usually craft anything but scroll text in a documentable period form, but this question came up on a list I’m on, which caused me to consider the question.
As a boilerplate, I’d look at this as a fill in the blank sort of thing:
This was inspired by ______ event. It is in ________ style (with detail – year, form, rhyme, alliteration, etc) which is appropriate because ____________. I wrote it to _______________. The tune is (original, a contrafact, a filk, etc – with detail if you can about the tune.) It has elements of _______________ (phrases in a different language, descriptions of events or places from a period document, etc).
For example, if I read Tristan and Islote, and I wrote a song about how much they love each other beside the sea as she nurses him back to health – I’d explain the story origin and its period nature, what inspired me about it, and how I’ve chosen to represent that in song. I’d talk about elements I may have borrowed from the original text (“Tristan say THIS THING” to Isolte, and that is in my repeated chorus” or “We know that the King will eventually learn of their love and I foreshadow that doom in THIS fashion…”) So an explanation of your literary understanding of the piece is good. I used the cantus line from a French church chant as the repeated chorus. It is from 1213 (and I’d include a picture of the original if I could.) Because their love is outside of the court, I choose to perform this without any instruments to reflect the simplicity of their love in that environment. (AND SO ON…)
That’s how I’d “document” an original work with period origins.