Here are some rules:
1. Write two versions: one (in the bass clef) where the tune is in the upper part and the second (in the treble clef), where the tune is in the lower part.
2. For counterpoint use the same rhythm as in the chant.
3. Aim for consonant intervals between the two parts: perfect octaves, perfect fifths, major and minor thirds, major and minor sixths.
4. Avoid two consecutive fifths and octaves.
5. Use contrary motion as much as possible.
6. When the tune is in the upper part, start and finish with a perfect octave.
7. When the tune is in the lower part, start and finish with either a perfect octave or a perfect fifth.
8. Aim for a smooth and balanced melodic line in the counterpoint voice - avoid two consecutive leaps in the same direction.
9. Avoid repeated notes in the second voice.
10. When you are done writing, play both versions on the keyboard to see if you like them.
Here is the PDF file for printing with two extra empty staves for writing in note-against-note counterpoint.