You must know all of your basic Triads before you can extend them to make them fuller. You will be surprised at how many musicians are not able to do this quickly.

When you play by ear it is important to have the numbers for every key memorized. These numbers will help you change key, create extensions, communicate with other band members, and help with notating music. This screenshot of the video above shows us the notes of the C Scale in order:

It is Very Important to know all of your scales for every key. Chords come from Scales.

When building chords, use the odd numbered scale degrees. The main triad would be 1, 3, and 5. The extensions would be anything after 5: 7, 9, 11, 13, and #11. Generally we do not use the 11 as an extension of a C Major Chord, since it confuses the ear and makes it sound like a dominant chord.

Generally we do not use the 11 as an extension of a Major Chord, since it confuses the ear and makes it sound like a dominant chord. See the two examples below

Avoid the 11th extension

The first note to add to your Major Chord is the 7th 

The resulting chord will be called a Major Seventh Chord. We usually just write Maj7 or M7 to indicate this a Major Triad + the Seventh Note. In the example below we added the 7th note (G) to an Ab Major Triad (Ab-C-Eb)

After that you add the 9th extension.

The chord is a Major 9 Chord. These are some sweet sounding chords!! Doobie Powell uses these voicings a lot in the song "Hallelujah, you have won the victory" In the example below we added the 9th note (Bb) to an Ab Major 7th chord

Notice in the example above that the notes do not have to be in the exact order as you see on the chart. Changing the order of the notes around is sometimes necessary in order to give "space" especially in the left hand when you do not want notes too close together