Let’s start by deciding how many bars long to make the main section.
Thinking about Rhythm
Count out your chosen metre, e.g. 1,2,3-1,2,3. Try to come up with some catchy rhythms that you might like to use that fit in with this. Mix and match minims, crotchets and quavers to discover interesting beats.
Beginning the Harmony
Let’s establish your home key. If you’ve chosen C major for example, play the notes of that chord to a nice rhythm. How many bars would you like to stay on this chord? You could have a longer melody that stays on this chord so that when it changes there’s lots of contrast, or you could move away sooner to build up a chord progression.
Harmony continued
Now we know what key we’re in, which other chords are you going to have in your opening section? This is where your piece steps away from home and ventures into new territories. Remember we still have to add the melody; this is just to give us the general feeling, like a base layer in a painting.
Finishing the Harmony
When you’re approaching the last bar of the main section, it’s time to think about returning home. Remember the chord built from the 5th degree of the scale is dominant and wants to lead us back to the home chord, so you can always use it to get you back to where we started at the key chord.
Melody
Once you have all you chords laid out on your manuscript paper, play through them several times. You may find that a melody will pop into your head as you do this, especially if you think about the rhythms you clapped earlier. Try making distinct phrases that don’t last too long. Often short and sweet is better than long and rambling. You can also use a question and answer technique. The first 2 bars could be a phrase that sounds like a question and goes up, the next 2 bars is a phrase that answers and goes down.