Here's the method I use every time I want to capture an idea. I draw out 8-bars (or measures) first. Why 8 bars? Because it is an ideal framework to work in. Eight bars of music are enough to generate a complete musical sentence and can usually be repeated once or twice. Next, I improvise and see what comes up. THEN I will write down the chords I am playing and the first 2-bars of melody.

Writing down the first 2-bars of melody helps me remember the entire theme for the 8-bar phrase. I usually stay within one key to make it easy. This means I'll have 6 chords to work with. In C Major, the chords would be, C Maj. - D min. - E min. - F Maj. - G Maj. and A min. This is more than enough material to work with. In fact, I rarely use more than 3 or 4 chords for the first 8-bars.

Once you get your first 8-bars down, your more than halfway home. Why? Because you already have the beginning. The rest of the piece, if there is a rest of the piece, can be finished by drawing more bar lines AND LISTENING FOR THE NEXT SECTION OF MUSIC. This is always accomplished through improvisation.

Your best material will ALWAYS come from improvisation because you are not thinking about creating something. Instead, you are allowing the music that is inside of you to come to the surface without forcing it or willing it into being. You use the 8-bar framework to hold your ideas so to speak.

There is no rule that says you must work within 8-bars. You can use 4 or even 16 bar phrases, however, its good to be able to feel the form and structure of an 8-bar phrase first. It is the structure used by most composers and it is wise to understand it.