in the most basic way you just keep stacking thirds on top of you chords so cmaj7 is CEGB, if you keep adding thirds you get the extensions cmaj9 = CEGBD, Cmaj11 = CEGBDF, Cmaj13 = CEGBDFA.
It's also interesting when you alter the upper extensions ie #11 on a major chord, a b9 on a minor...