Extended chords are simply chords that contain added intervals above the seventh. In order to effect an extension, simply count the thirds to the desired interval and “add” that note to the chord structure. To illustrate this, take a Cmaj7 (C-E-G-B) and stack another third onto the seventh, which would be a D, this would effect the ninth, making the chord C (1st), E (3rd), G (5th), B (7th), and D (9th) or Cmaj9.
Do not assume that all of these types of extensions will occur naturally. For instance, in the song Cold Shot, Stevie Ray Vaughan plays an E7#9 (VII – E-G#-D-G). At first glance, this appears to be an inversion but is actually a dominant E chord with a flat seventh (the natural seventh would be D#) and an extended sharp ninth interval (because the natural ninth interval is F#). These type of extensions do occur now and again, and one should be cautious not to presume that every extension will appear as it does in the major scale from which it is constructed.