This Gradual is based on the opening of the modified Gregorian chant melody "Unam petii a Domino". I thought I would share with you the beginning of this piece because it works perfectly as a harmonization exercise.
So in the above picture you can see this melody of 8 measures. Try to harmonize it with 4 part harmony observing the rules of voice leading and avoiding parallel fifths, octaves, and movement of all parts in the same direction.
I have harmonized this melody using root position, first and second inversion chords as well as the D7 chord at the end. Below each chord I have written the chord symbols so that you will see exactly what kind of chord I used for each instance.
See if you can create a different harmonization than I did (there are dozens of ways to do this).
Here is the PDF file for printing and the MIDI file for listening.