Actually, when I started to learn theory for guitar years ago, modes were some of the first things I learned, as they are quite important for modern rock/metal guitar. I understood the concept, but didn’t know how to apply them properly until I started with learning orchestral composition.
In essence, the basic “Greek” or “Church” modes can be thought of as scales within scales. Basically I learned them by taking any regular major scale and then playing the notes of that scale but starting on a different interval of that scale. The modes in order are: Ionian, Dorian, Phrygian, Lydian, Mixolydian, Aeolian, Locrian.
So, using C major as an example, if you play the notes of C major, but from D, you have a D Dorian scale. If you start on E, you have an E Phrygian. Then F Lydian, G Mixolydian, A Aeolian and B Locrian. Note that the Ionian mode and the Aeolian mode correspond to the regular major scale and the natural minor scale respectively (C Ionian = C major, A Aeolian = A minor.)
Each mode, even though you’re playing the exact same notes, derives it’s unique sound from the different whole-step, half-step patterns associated with that mode. In this sense, each mode has a specific set of intervals that give it its character. For example, the Mixolydian pattern is almost identical to the major scale, but it has a flat 7. The Phrygian has a b2, b3, b6 and b7. The key to getting a modal sound is to focus your melody/harmony on more of the intervals particular to that mode.
I’ve only real recently started to use more modal harmony; my Star Trek tribute uses Mixolyidan and my “fantasy music” piece uses a whole mix of different modes, but focuses mainly on the Dorian mode. I’m currently writing a hybrid Scfi-Fi action piece that uses Mixolydian as well.
Hope this helps a bit!