Analyze the song. Look at how fast or slow the theme progresses. A fast theme may have few chords, while a slow theme may have several.
Count the number of measures in the song. This step will make it possible to determine how many chords you will have in the song and where to place the final cadences. If there are four measures, you will ideally have at least four chords.
Decide where to place the chords. You have several options, you can place the chords at the beginning of each measure, every other beat, or on the main accents of the piece. This decision is largely a creative decision left to the composer.
Determine what key the song is by looking at the starting and ending pitches as well as any accidentals that occur in the song. If the theme begins on an A, ends on an A, and has three sharps, then the song is in A Major.
Insert a dominant seventh chord in the penultimate measure. This will help to give the theme a sense of finality. A dominant seventh chord is built on the fifth scale degree of the key. If you are in the key of A, the dominant seventh chord will begin on E. Build the dominant seventh chord by creating a major triad and adding a minor seventh to the top.
Insert a tonic triad in the last measure. A tonic triad is a triad built on the first scale degree. This will provide your theme a return to the initial chord. In combination with the dominant seventh chord, this will create a strong resolution to end your theme.
Decide on the chords for the remaining measures. Any chords are acceptable, but do not move from one scale degree to an adjacent scale degree. For instance, a chord built on the third scale degree can move to the fifth or the first scale degree, but not the second or fourth.