When a landscape artist wants to get the essence of a beautiful scene, they make a quick pencil sketch of it.
The artist doesn't want to represent the entire landscape as it is, but as they feel it to be. A few lines scribbled here and there indicate the feelings the artist wishes to convey. We musicians can do the same thing! We can quickly sketch out our ideas on paper by using chords and a chord chart.
A chord chart is just a way to notate when chords change through time. You can write out 8-bars to begin with (as I do). Now let us suppose you have the urge to capture something musically. What do you do? Well, for starters, you can pick a key to compose with.
For example, let's choose the Key of C. Now after deciding that, we know that the piece will have a Major sound to it. We also know that we have 6 chords to create with from the C Major scale. With these primary decisions out of the way we now can focus on notating our musical ideas and capturing a mood. Here's how I do it:
I start by just improvising and letting the music come out naturally. For example, I play a C Maj. 7 chord and I like what I hear. I'll write down the first 2-bars of the melody, then place a chord symbol on top of the chart so I now have the musical idea in place.
My goal is to finish charting the 8-bar phrase with chords all the way through. Once this is accomplished I have the first section of music. If more is to come, I simply write out another 8-bar phrase and keep adding more music.