Select the text you will use for the basis of the cantata. You may select text from the gospels of the Bible that deal with Christ's birth, find a text from a Christmas poem (be sure to ask for permission from the author), or you may write your own text.
Compose a melody (hymn) that will serve as the musical theme for the cantata. The melody may use any notes or rhythm you like, but it should use not use intervals of more than a 5th and should be primarily stepwise (e.g., C to D). Keep the melody within 1.5 octaves.
Decide how you want to deliver the text you have selected. For example, what pieces of text are better delivered as recitative (text is sung with very little accompaniment with minimal melody, as if reciting)? What texts seem suited to just one singer, or lend themselves well to a choral setting? If you choose soloists, do not extend beyond three arias and recitatives at a time between choral settings.
Decide what kind of voicing you want for and supporting the text. For example, if you would like a soloist, will they be a soprano or bass? Will the choir sing in standard 4 part harmony, or will you experiment with double choruses, divided parts, or antiphon (call and response melodies, rhythms, or harmonies)? Will you have just piano accompaniment or will you hire a small orchestra?
Sketch rough versions of each section of the cantata on staff paper or in Finale.
Flesh out your sketches and make sure that all sections transition well. Check the score for any errors in text or notation, as well as in voice leading.
Write or print out individual parts as needed based on your orchestration (e.g., piano part for the pianist, vocal/choral score for the singers).
Find musicians who are willing to perform for free or for a fee, depending on your budget. Locate a venue in which to produce the cantata, such as a local church.
Rehearse the cantata and work with volunteers or hired personnel to handle any technical aspects (e.g., lighting, microphones), produce costumes, and build a set. (Your cantata may be completely unstaged or as dramatic as a full opera, depending on your vision.)
Advertise the cantata and perform it on designated dates.