Choose a starting note for your chord. If you choose a C, it becomes the root and determines the name of the chord. The 9th chord will be called a C9, Cmaj9 or Cm9, depending on the notes you choose for the remaining intervals. Play and sustain the root note.
Count three or four half steps to the right of your starting note. Half steps move from one note to the very next, including the black keys. Three half-steps form a minor 3rd interval, and four half-steps form a major 3rd interval. This note is the 3rd of your chord. Play and sustain the root and 3rd of the chord.
Count seven half steps to the right of the root note. This note is the 5th of the chord. Play and sustain all three notes. You now have a major or minor chord, depending on which note was chosen for the 3rd interval.
Play the note three or four half-steps to the right of the 5th note of the chord to form a 7th chord. Play all four notes together to hear a minor 7th, dominant 7th or major 7th chord. Playing the note three half-steps to the right with a minor chord is called a minor 7th. It is called a dominant 7th when played with a major chord. A major 7th is formed when a major chord is played with the note four half-steps to the right.
Count seven half-steps to the right of the 5th of the chord. This note is the 9th of the chord because it is a 9th interval from the root note. Play the 7th chord with your left hand and the 9th interval with your right hand. The chord formed is either a minor 9th, dominant 9th or major 9th, depending on which 7th chord is played.