Here is what I did with this melody in order to create an organ Offertory:
1. I took the last 11 notes of the melody and created a refrain out of it and harmonized it in parallel 7th chords in mixed position (played on the foundation 8' stops).
2. The chant phrase appears between each refrain played by the right hand on the combination of 8' and 2 2/3' stops.
3. To increase the variety in the refrain itself, I start it on the last note of the preceding phrase of the chant.
4. To keep the unity in the piece, I use the mode of this chant throughout.
5. The pedals play long pedal points on the main notes of the mode.
6. Although the caesura signs in the Gregorian chant notation have 3 different levels of breathing points which should be treated differently in term of rests, I chose always to have eight-note rests.
Here is the video and the PDF file for printing if you want to play it.
By the way, if you are wondering, how to read the notes in the chant notation, it's quite simple: on the top line there is a C clef (indicating middle C), which means that the starting note is tenor F.
Sometimes there is a flat sign next to the note which is valid for that particular note (Bb). The dots, stems, and the dashes make the notes longer. Of course, there is a lot more to it than knowing the notes, but it's a good start.
NOTE: the entire melody could be transposed to any pitch level, of course, but the interval relationships should remain the same.
I made this, you can make it, too.