This video shows how chords are contructed from
the 7 notes of the Major scale.
Chords are built by initially tacking
3rds upon one another. Or, by taking the root note and
adding every other note until you have 3 notes for a basic
chord or triad.
For example: A C major scale has the notes C -
D - E F - G - A - B C
To create a chord with the C note as the root,
we add every other note to get 3 notes... C E G. That's your
basic C Major chord.
You then do the same with the rest of the notes
in the scale.