The documents are written on vellum. Work started by drawing lines that defined where the various parts should come.
Double cross of line, left vertical line for the clefs, right for the text.
The staves where painted first in red. Then the text was written where sufficient space was left to write notes afterwards.
Because of the defined way of working for an original document it is possible to spot also how the new text was added later. The change started by scraping of the to be removed text.
Start of the line at the verso side. Original letters vaguely visible
Then the new syllables where written underneath their starting notes.
Notes where added to make it possible to sing the text.
At last the new words where written, sometimes not fitting within the guide lines