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:
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
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
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.
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