Altered scale is made by the sequence: semitone – tone – semitone – tone – tone – tone – tone.
We already said, in the article “Melodic Minor Scale” that altered scale of a chord can be made through a melodic minor scale one tone above this chord. For example, the altered scale of G is the melodic minor scale of G#. This makes our life easier, because we already know the melodic minor scale.
Learn more here: http://www.simplifyingtheory.com/altered-scale/