@PeterTurner Hearing the sample Gregorian chant performance of Ecce panis angelorum with simple supportive positive organ accompaniment whose poetry turns out to be written by Thomas Aquinas (according to Hymnary.org) makes me want to learn how to read the notation and how to accompany it using a simple positive organ (like in this tutorial).

If you DO want to use it for your procession, I found the equivalent modern notation of the first 2 verses of the chant here. Good luck!

(BTW, thanks to @KenGraham for bringing the above to my attention via his answer).