Chords are based on the intervals of a scale. The major scale contains seven intervals including the octave. Each interval of a major scale is two half-steps apart, except the interval between third and fourth and between seventh and octave. Start building your chord from a root note.
Add the major third, two intervals above the root note or four half steps. For the C major scale, for instance, your third would be E.
Add the fifth interval to your chord. Using the C scale as an example, if your root and third are C and E, your fifth will be G. You can also include the octave of your root note.
Minor chords are constructed in similar fashion to major chords, but use different intervals. As with a major chord, begin with a root note.
Minor triads, the most common type of minor chord, diminish the third by a half step. In a C chord, the normal third of E would be an E flat. In any scale, you reach the minor third by moving three half steps up the scale from the root.
Add the fifth as you would with a major scale. Fifth intervals are always four half steps above a minor third.
According to Sound Feelings, an augmented chord requires that you take the standard major triad of any key and raise the fifth by a half step. In the key of C, the fifth should change from G to G sharp, making the entire chord C, E and G sharp.
Diminished chords exaggerate the effect of a minor chord. To diminish a chord, lower both the third and fifth of a major scale by a half step. A diminished triad for a C chord would be C, E flat and F sharp.
According to Guitar Secrets, seventh chords require you to add another note to your chord. Instead of playing three notes, you'll play four. Play the major chord as normal, and add the seventh interval. Starting with C, you would play E, G and B in addition.