1. Place the tune in the soprano and supply missing 2 middle parts
2. Use 3 note chords (tonic, subdominant, dominant etc. and their inversions)
3. Dominant seventh chord works well for the ending of the phrase.
4. End phrases in E minor, A major, G major, D major, E minor, and G major.
5. Make sure you avoid parallel fifths and octaves by moving the upper 3 parts in opposite direction than the bass
Take a slow tempo, aim for detached articulate legato touch and 3 correct repetitions in a row. Here's the MIDI file of soprano and bass parts for listening.
Post time and the number of repetitions to comments.
[HT to Pierre]