All "interesting" harmonies you might conceive should only be reached through individual voice-leading, rather than arbitrarily. I think this way even when extemporizing organ accompaniments.

Perhaps a word more pleasing word to Schönbergian would have been "engaging". As a mostly-tenor-but-sometimes-baritone/bass I've had my fair share of boring parts. Switching to bass suddenly got a lot more fun.

This reflection leads me to another thought: although it's something that can't always be avoided, I also try not to have my singers stay at either end of the extreme of their ranges all the time either. As a tenor I absolutely dread Handel's Alleluia because he has the tenors scraping the empyrean heavens the entire time... it's [literally] exhausting.

the tenors scraping the empyrean heavens the entire time

Wow, I just looked at the score---how right you are.

I usually have found Brahms difficult to sing, mainly because of lack of coherence in the (first) bass. (And the misery of the texts).

I second MJO's recommendation to study the chorales of J.S. Bach. CPDL has the following collections:

Chorale Harmonizations (BWV 250-BWV 438)

185 Bach Chorales

389 Choralgesänge

@Anna_Bendiksen
I generally find studying music videos with good audio, and understanding the relationship between the individual voices the most comprehensive way for this.
The other: write, and keep on writing.

Going back a ways in time, have you studied this yet? https://www.amazon.com/Study-Counterpoint-Johann-Joseph-Parnassum/dp/0393002772

book is free as a pdf here:

UPDATE

(apparently still in copyright... link deleted)

I second the motion on the Bach Vierstimmige Choralgesänge (read through them all on the piano if you are able) and Fux (note-against-note, two-notes-against-one, and "ligatures" are the 3 species that stayed with me from when I did some).

It is sometimes said that "A composer who sets his own texts has a fool for a collaborator." That doesn't have to be (and is probably not in your case), but it very often is. Verse is a separate skill set from composing, and because we speak language from infancy and not music, we tend to think we have it covered. Before you set your texts, turn a hairy eyeball to them and ask: does each verse scan? (exactly, as in hymns, not approximately as in the OCP School). Are the rhymes forced? (might be better to rhyme less, or half-rhyme). Is word order awkward? Are you using archaicisms solely for metric or rhyme purposes? (If you're trying to create an archaic world, then you'll be policing for the modern.) Are there words that seem stuck in just to fill the meter? Is every word earning its keep, every adjective, adverb, noun and verb the most powerful possible? In short, does the poem sing with its own music? If so, it's ready for your music.

Next, you need a tune. Common pitfalls: 1)stultifying rhythm of quarters and halves 2)cadencing only on tonic and dominant 3) too much internal repetition, or not enough. Your tune will probably (but not necessarily) have some kind of climax toward the middle of the 3rd line of a 4 line stanza. Be aware of what the "hook" is; even hymn tunes have them.

Next, you need harmony. A collegiate Harmony 101 course would teach you everything you'd need to know. It's also kind of the "breaker" course, that separates wannabe music majors from real music majors. James Dapogny, late of the University of Michigan, had a "table of offenses against tonality with their penalties" and would objectively grade 4-part chorale exercises, and I earned more than my fair share of red ink. But almost everything in music is nascent in chorales, and if you master that style, you have a foothold on everything else. You can do it with will and a copy of Aldwell and Schachter, but it's a lot easier with a friend to keep you honest. Besides "the rules" (no parallel/direct 5th-8ves, double leading tone, bass motion over barline etc.) consider these positives:
1. Use chord inversions. 6/4s are faithful servants as passing chords and hard masters as downbeat phenomena, 6/3s add buoyancy and make your bass line more melodic.
2. Use tonicisation/secondary dominants. V/vi is particularly useful, and "V of" doesn't necessarily need to go to the "of"; it can go to vi/"of" or to "of7" (V7 of IV of your "of"). Don't be a three-chord hymn composer!

Hope this helps.

I wouldn't bother with Fux in 2020 - due to the lack of published Renaissance works in his lifetime, the method was only able to loosely approximate the Renaissance style. Knud Jeppesen's text is a much better option for learning 16th-century counterpoint. As for the 18th-century, there is no better way to learn than studying the works of the North German masters.

I would recommend both Fux and Jeppesen. Of the two the latter is the most scholastically significant discussion of renaissance practice (Palestrina in particular), but Fux is a staple practicum which everyone should know and profit from.

does the poem sing with its own music?

in addition to Jeffrey's comments, I would suggest that your hymn also be prayerful. I've found this to be a particular trait of the hymns of Faber, Caswall, Procter, Newman, Donnelly and many others whose hymns were first published as text only and then composers would set them to melodies. Today I think hymn writers compose their words with a particular melody in mind a luxury early hymn writers did not have.

Anna,

I wrote a hymn text and the music to go with it, which I sent to my mother ( a musician of some worth, but not very much note, who studied at the Royal College of Music back when Lloyd Weber was the father, not the son) who sent it to my aunt (her sister-in-law) who commented that while she liked the text very much she had felt the music unsuited to it, and had written her own, and included it in the envelope. I was much younger then, much less experienced, and I haven't looked at the text or the music in years, but I will always remember the feeling of "that music didn't suit the text, so I wrote something better". I'll see if I can find the stuff if you're interested.

About composing: think like a singer if you're writing for voices, not like a music theory student. Singers need to breathe, but music theory exercises don't. Singers like their lines to make sense, within the context of the piece as a whole as well as in isolation. Remember that (the Bach recommendation notwithstanding) not every harmonization works for voices which works for organ, or non-organ instruments. Human voices have ranges, and these should be respected.

Fux and many other schools are good to study. They will ALL be a help in leading you to find your own voice as each becomes a building block toward mastery. If you are able to take in theoretical listening studies, it can be a much higher plane in the absorption of compositional technique. This means that you visualize the music manuscript as you hear it.

Wow, Richard.

Can you give us a short synopsis of that?

I would love to see what Mozart would do with some of my own chromatic writing… lol.