Choose the key in which you're going to play your song (or instrumental piece). Knowing the key is necessary to finding the right chords to the music you're playing. The first note of the scale is called the tonic, and the root will be later used as a reference point to finding the primary chords or any other chords.
Determine the accidentals in the key signature. This step requires some basic note reading. The key signature occurs at the beginning of the music, and it contains either sharp or flat signs. These signs tell the performer to always play these notes as either sharps or flats unless they are canceled out by a natural sign. Once you've determined the tonic, you can consult the circle of fifths chart to find the accidentals of the key signature. (See Resources)
Find the first fourth and fifth degrees of the scale. These are the notes that we will later construct chords of three and four notes. The first degree is the same as the tonic. When finding the fourth and fifth degrees of the tonic, count up four and five scale degrees, respectively. For example, in the key of C the fourth degree of that scale is F while the fifth is G.
Construct triads (or three-note chords) on the first, fourth and fifth degrees of the scale. Starting on the tonic add the third and fifth degrees of the scale to get the tonic or "one" chord. For example the one chord in C consists of the notes C, E (the third) and G (the fifth). Repeat this concept on the fourth and fifth degrees of the scale to get the IV and V chords. Remember, the key signature of the tonic when constructing the IV and V chords.
Add the seventh degree to the V chord in the previous step to get the often used five-seven chord (V7). Start with the fifth degree of the key your choosing and count seven steps from the fifth degree and NOT the tonic. From the last step, we determined the fifth degree of the key of C is G. Starting from G count seven steps up from G and we get an F. Add F to the G chord to get the four-note chord of G, B, D and F.
Play the inversions of each of the three primary chords. An inversion of a chord means another note besides the root of the chord is on the bottom. Take the bottom note of the chord and put it on top. For example, a C chord with the notes of C, E and G can be inverted to E, G, C and G, C, E. Each three-note chord has two inversions while the four-note V7 chord has four separate inversions.