Use these tips to help you with your angle binding.
To create a mitered corner binding on the 120 degree angle, sew the binding over the first side stopping a quarter inch from the corner.
Fold the binding back on itself until the strip is straight to the next side. Notice how the binding is folded continuing in a straight line from the next edge to be bound.
Maintaining that fold, bring the binding over itself to lay over the next side. Secure the corner folds with a pin or clip.
Use this trick for all outward angles.