• basic chorale-style (SATB) chord voicings will serve as a foundation for additional color
• they also allow us to evaluate harmonic choices
• we have soprano and bass; now we just need alto (right thumb) and alto (left thumb)
• since we have the root already, we will add 3rd and 7th for tenor and alto
• use 5th if the melody features 3rd or 7th
• we will start by assuming diatonic chords: all notes in the key
• if any 3rd, 5th, or 7th conflicts with the melody, adjust it in the chord
• assign these notes to tenor and alto voices in whatever order makes sense
• try to keep the tenor and alto lines smooth where possible
• avoid putting tenor more than an octave below alto
• move the bass note up or down an octave as needed to make it playable

Other resources:

• Basic Music Theory / Chords: Triads, Seventh Chords