Decide on a melodic type, which will dictate how many notes will be sung on each syllable of each word of the text. The three melodic types are syllabic, which uses one note per syllable; neumatic, which uses two or three notes per syllable; and melismatic, which ranges from around six notes per syllable all the way up to 60 in more complicated chants.
Decide whether your chant will use recitative or free melody. According to “Gregorian chant - History, Musical form, Performance, Liturgical functions, Influence,” these are the two types of melody used in Gregorian style. "Recitative" repeats the same group of syllables throughout the chant, often with different notes used throughout the song, and "free melody" varies both the syllables and melody used.
Determine if you would like your chant to be liturgical or secular in nature. Liturgical chants are for public religious services. They are limited in appeal and may not be embraced by religious figures, but a custom-written chant can make a person's faith more personal than a chant chosen from a book. Secular chants are chants in which the text is not of a religious nature.
Pick words for your chant. According to The Gregorian Association, Gregorian chants are typically in Latin. Find a Latin translation of a personally meaningful saying or phrase. Pick a short phrase if it's to be repeated, or a long phrase if it's to be part of a free-flowing melody throughout the chant. The lyrics of your chant will be easier to remember and more personally meaningful if you understand them, so get a translation.
Find a succession of notes to which you can sing the words of your chant. Gregorian chants typically feature a system of reciting tones that repeat with slight alterations in the course of the chant. Find one that fits well with both your chosen words and your chosen melodic type, then try it out with a group of singers.