1. Make sure instrument is tuned to the proper pitch. Hold it in playing position. 
2. Simultaneously fret the G-string on the 1st fret (left-hand index finger) and 15th fret (right hand thumb).
3. Check the amount of clearance between the bottom of the string and the top of the 7th fret. (While still using right hand thumb to hold the string at the 15th fret, you can use the index finger to lightly tap at the 7th fret to more clearly see the gap). There should be approximately .003 to .006 inches-that is about the thickness of a couple sheets of copy paper. 
4. If the gap is too large, tighten the truss rod (clockwise). If the gap is too little, loosen the truss rod (counter-clockwise).
NOTE: For guitars that adjust at the neck joint we use a 8" long 3/16" wide blade screwdriver to adjust the truss rod nut. We sand the sharp edges off the blade of the screwdriver to minimize scratching the guitar.
Hold the screwdriver at a 45-degree angle to the plane of the body. Insert the screwdriver into the screw head at the neck to body joint. Keeping the screwdriver at a 45-degree angle, push or pull it like a lever. Make sure you feel it seat into one of the four sections of the screw head before moving it. Hint: If you get in a position where the slot is disappearing below the body horizon and the next slot has not yet appeared enough to get the blade of the screwdriver into it then put the screwdriver (at a 45 degree angle) into the slot that is disappearing and twist your wrist slightly in the direction you would like it to go. This will move the screw head just enough for you to get into the next slot.