i think of it this way. c ionion, d dorian, e watever, f watever, g mixolodian, a aolian, b locrainian. all have the same notes. but the notes play different roles.

for a simple example, if u are playing without backking track and u tend to play the note c a lot, it will sound like you are playing in c ionion. if u tend to play the note a a lot u are playing in a aolian.

another example. if u are playing over the chord progression C majour, G7 then A minor. u should play C ionion, then G mixolidian, then A aolian. the notes are the same for all but u should play them differently. playing the root, 3rd, 5th (and seventh for the G7 chord) will make the lead sound like it fits with the chords you are playing over. but this might be a bit boaring sounding. playing too many 2nds 4ths and 6ths will make the lead seem to not fit quite right. but using some will make the lead more interesting.

this is just the beguinin.