The ninth chord is created by adding the ninth note of the major scale to the dominant seventh chord.
i.e., Major chord formula: 1 3 5
Dominant seventh chord formula: 1 3 5 b7
Ninth chord formula: 1 3 5 b7 9
e.g., the C9 chord is based upon the notes in the C major scale.
C Major Scale
Here is a common movable shape for the ninth chord.
C9 (Root 5)
Deaden the low E string by touching it with the tip of the second finger or the thumb of your left hand.
This exercise uses all root 5 chords.