1. Angle the red lines outward in step 9 (narrower at the nose, wider at the tail).

2. Angle the black lines inward in step 9 (make the wingtips narrower at the front, wider at the back).

3. Add some upward flaps at the tail (worst solution).

1. Widen the angle of the wingtips (from 90 degrees to 135 degrees).

2. Add some downward flaps.

1. For minor imbalances, tilt the body left or right relative to the wings (in the direction where you want the plane to go).

2. For moderate imbalances, widen the angle of one of the wingtips (the one on the opposite side of where you want the plane to go).

3. Serious imbalances are probably due to an asymmetric fold somewhere (e.g. one wing is longer than the other). To save time, it's probably easiest to just toss the plane in the trash and start over. The symmetry on folds 1, 4, and 9 are crucial.