Determine the key signature by using the circle of fifths. You will need to know the key to determine the underlying chord progression. Simply count the number of sharps and flats in the key and then determine what key signature it matches. To determine major or minor, look at two things: the first note of the melody and accidentals. The first note of the melody will usually be the name of the key. A raised seventh scale degree indicates minor. For instance, in A-minor, the G would be written as G-sharp.
Find all of the tonic chords in the melody. The tonic chords start on the first scale degree. As an example, in C major, the tonic triad has the notes C, E and G. Label the first beat of these chords with a line through the music to indicate the starting point of the chord.
Find any dominant, mediant or subdominant chords in the melody to locate cadence points. Dominant chords start on the fifth scale degree and subdominant chords start on the fourth. The mediant starts on the third scale degree and only appears before the tonic in minor keys, when used as a cadence. In the key of C, a dominant chord starts on G and a subdominant chord starts on F.
Look for a transition in the chord progression from a dominant or subdominant to a tonic chord. The point at which the tonic chord appears after a dominant or subdominant phrase, marks the end of the antecedent phrase.
Find the end of the consequent phrase by finding the next point in the melody where the dominant or subdominant returns to the tonic.