I got a decent grade on it. The only mistake indicated by the professor was a point where I let the two parts reach unison that wasn't the final cadence. There was a lot of rule-following and not much creativity, but I had fun with it. The professor made it clear that this was not a creative assignment, but an exercise in understanding the rules of 16th century counterpoint.
The professor gave us a simple strategy for composing the piece, and I tried to follow it without deviating. We were to first decide on the length, roughly 32 bars. No accidentals in the key signature, 4/4 time. We needed two internal cadences and a final cadence, each of which needed to be on a different mode. Sample one-bar cadences were provided, and we were encouraged to lift them directly, so I did. The interaction between the two parts was to be imitative, meaning one part starts an idea and the other is modeled after the first, emulating it until the cadence. The dactylic motive was to appear at least once: one whole note followed by two half notes, or one half and two quarters, or one quarter and two eighths. Construction of the lines needed to adhere to contrapuntal rules for melody and harmony: lots of stepwise motion and lots of 3rds and 6ths in the harmony.
I had a pretty easy time in Finale. I just wrote a long single line sticking to the melody rules, copied it to the other part, and displaced it a few beats or measures. Then I examined the intervals between all the notes and tweaked any notes that needed it in the new part. We didn't need to think at all about progressions or implied chords, just strong melodies and consonant intervals. So if it sounds like it's lacking a coherent progression, that's because it is.
More work samples on the way.