If you want to learn how to arrange, you have to practice writing chords. In my classes I often find that writing chords over four staffs is difficult for aspiring arrangers. Therefore, I devised some easy exercises. First, here is an exercise for writing triads.
The exercise looks like this: 
Each system has four staffs for the voice groups (soprano, alto, tenor and bass) plus two staffs for piano. The idea is to write four voicings for each chord. As an example, two voicings are given for the chord G and in the next bar one voicing is given for Dm. The chords do not have to sound good in this order. So, the exercise is a purely vertical one; just write good chords. So you might start filling in the chords like this:
Here is the full sheet with exercises as a pdf file. And here is a possible working-out of the exercise. In the answers, I wrote down just a few of countless possibilities of course. But maybe the answers will inspire you to write even different voicings.
Here are some additional rules for doing this kind of exercise:
- Work at the piano, so you may hear the different sounds of the voicings.
- Make sure the chords are complete, meaning that the root, the third and the fifth must be sung in at least one of the voices.
- Write the soprano and tenor between C and G and the alto and bass between F en C. Thus, the bass may not be below F3.
- Don’t write crossings between the voices. For example, make sure the alto is never below the tenor.
- The lowest note has to be the root of the chord. If you would write another note than the root, we would be dealing with a slash chord, that is a chord with an alternative root written behind a slash.
- Beware to write the tenor in the right clef. The tenor sounds an octave lower than they seem to be.
- For nice pop and jazz chords, preferably keep the top three voices rather close. (Wide chords tend to give a more classical sound, which is less suited for pop and jazz.)
- Notate the voicing both for the choir and the piano.