1. Place the tune in the soprano and supply missing 2 middle parts.
2. Use 3 note and 4 note chords (tonic, subdominant, dominant etc. and their inversions).
3. Dominant seventh chord works well for the ending of the phrase.
4. Make sure you avoid parallel fifths and octaves by moving the upper 3 parts in opposite direction than the bass whenever possible.
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.
PS Today I'm playing an improvisation recital based on the legend of "Jurate and Kastytis" in my church.