Modes are very easy to understand, conceptually.

If you know the C major (Ionian) scale, it goes C D E F G A B C.

To play D dorian, just start on D and play: D E F G A B C D. Same notes, just starting on a different note and going up.

All you're doing is "inverting" the scale.

If you shuffle them around enough you'll see that C Ionian and F Lydian are the same notes.

You'll also find which notes you change to get to a new mode. For instance, C Ionian... raise the 4th (F) and you have C D E F# G A B C.

Then it's just a matter of memorizing them, learning the sound of them, figuring out how to get between them and use them.

Schlegel has a 24-lesson Tutorial on Modes here: http://www.guitartricks.com/lesson.php?input=13423

-Carl.