Voice-leading is the way the different parts work together. These are the main things you need to worry about in order to make your composition stylistic:
S
Spacing and Doubling
Chords are usually spaced with the largest gaps at the bottom (i.e. between the lowest parts) unless the aim is to create a darker, murkier texture. In a basic MDH texture your bass instruments will usually have the same line to avoid murkiness.
Parts avoid crossing unnecessarily (particularly above a melody at the top of the texture)
It is normal to include all the notes of a triad or seventh (although the fifth or the third are sometimes omitted in the latter)
The most common note to double in a chord is the root
L
Leaps
Avoid unnecessary leaps in melodies (a dramatic leap in a melody is fine – a random leap of a ninth in the viola part is probably not)
Avoid two large leaps in the same direction
A
Augmented and diminished intervals
In melodies, be aware of augmented and diminished intervals (particularly augmented seconds and tritones) – if you overuse them your melody will sound odd.
P
Parallels (5ths / 8ves)
Avoid consecutive parallel octaves and fifths, particularly between the melody and the bass
Octaves used to double a melodic line or the bass line as reinforcement are fine
