How to Harmonize Modes.

Diatonic Modal Chords

Ionian Mode (I)

In modern time and usage, the Ionian Mode is the same as the Diatonic Major mode. 

Diatonic Ionian Chords

Dorian Mode (II)

Some Dorian themes are based on a single chord and modulate on another l-7 placed above the first one. To avoid the monotony, due to the presence of a single chord, one often uses fourth voicing. "So what", by Miles Davis and "Impressions" by John Coltrane are composed this way. Please note that none of the modes below have a Major (V7) and thus no dominant seventh chords.

Diatonic Dorian chords

Phrygian Mode (lII)

The Phrygian mode is the old D mode in the Ancient Greek modes and has become the E mode in the middle age. Characteristic note= b2

Phrygian Diatonic Chords

Lydian Mode (IV)

Characteristic note c=#4
Lydian Diatonic Chords

Mixolydian Mode (V)

Mixolydian Diatonic Chords

Aeolian Mode (VI)

The Aeolian mode is also called the natural minor mode (-Nat). It has no leading tone. So the (V) degree, can not be used to resolve on its Tonic. Its Tritone being placed between the degree (ll) and degree (VI). 

C natural minor scale and its relative Major scale
Aeolian Diatonic Chords