There are several methods for learning the modes. I tend to think of the modes in this fashion.
There are 3 major modes and 4 minor modes.
First, learn the major modes.Ionian Lydian Mixolydian
Play these modes over a G Major backing track and see how the major modes sound.
Next, learn the minor modes.Dorian Phrygian Aeolian Locrian
Play these modes over a G Minor backing track and see how the minor modes sound.
More information on how to learn the 7 modes can be found here:Learn All Seven Modes