These following simple rules or tricks will always work in most circumstances. I recommend doing the paperwork. Understanding these rules, will give any serious guitarist a very clear understanding as to how modes work in American or blues oriented music. Some of the following ideas break traditional logic. All can be started from the third fret, ONLY! Play a simple G bar chord as background or loop.

Play a major scale from:

Ionian - Root (for G play Gmaj scale) I

Dorian - b7 or whole step down from root (for G play Fmaj scale) ii

Phyrgian -m6 above the root (M3 below the root) (for G play Ebmaj scale) iii

Lydian -5th above the root (for G play Dmaj scale) IV

Mixolydian - 4th above the root (for G play Cmaj scale) V

Aeolian - m3 above the root (M6 below the root) (for G play Bbmaj scale) vi

Locrian -1/2 step above the root (for G play Abmaj scale) vii