The Dom 9th chord is the Dominant 7th with an added 9th note. The 9th is the 2nd note one octave higher..

Our ukuleles have a limited range of about 2 octaves so we fudge it a little and play the most convenient 9th note.

Note how the 7th is modified by adding the 2nd note of the scale of the root note: Play 3; 5; b7; 9(= octave 2)

In this case the root note is 'covered' by the 9 note. The root note becomes an 'implied' note. The ear 'hears' it! .

C9 = C7 D on C string D9=D7 E on C string E9= E7 F# on E string F9= F7 G on E string

the 9(A) is on Gstring Here the 9(B) is on Astring here the 9(C#) is on Astring

You can now look back at all the shapes for playing Dom7th chords and add a 9 note to discover the Dom9th