Im not sure if this has been covered but rarely does anyone discuss octaves. Learning modes in octaves is a good way of stringing it all together. It also makes it easier to see the degrees, whole step/half step thats required to make up each mode. The trick here is to play the 3 note per string method. So if your in G major, the lowest octave for this would be to play g, a, b / first, middle, pinky on the 6th string. c, d, e / first, middle, pinky on the 5th string), f#, g / first finger and a half-step slide 4th string. From there your all set for the next octave, just remember to allow an extra halfstep shift up for the f# , g notes on the 2nd string, and from there go on to the third octave, same notes, same shape but end with a f# tap/slide to g on fret 15. Economics come into play too in that you dont have to struggle to remember that many shapes, obviously if you start with the 2nd degree 'a' in this same shape you get the dorian mode and if you begin with 'c' still in this shape youll be in lydian, 'd' mixolydian and so forth.

ok, hope thats not too confusing.