Select a song that you find inspiring and meaningful. It should have a strong melody and interesting harmonic possibilities. If it is in the public domain, licensing is not required. Get to know the song intimately, and decide how you will harmonize each note.
Now that you have a rough sketch, you can think about ways to make it more appealing. Avoid keeping the voices in a narrow range, singing continuously in four parts. You might start simply, add fullness gradually, reach a climax, then wind down. You could experiment with humming, mono-syllables, imitation, an ostinato, or call and response. Experiment with some of the options below to introduce more contrast in your arrangement.
Examples of contrasting textures are dense vs. sparse, close vs. open, and active vs. inactive.
Change from homogeneous tutti to accompanied solo; feature a small group within the larger group; add additional lines or ornamentation when the music is repeated; put the melody in a different voice; double the melody in two or more voices; feature different parts of the choral range.
Change relative activity and complexity; use an ostinato; add or delete rhythmic layers; change tempos or time signatures; introduce syncopation.
Progressively add or delete voices to produce a wave, a pyramid, or a fan shape; use two contrasting pairs; contrast one voice with others; use parallelism or planing; combine different sections in diverse ways.