Here's one of my favorite methods for quickly blocking out entire sections of music and creating a complete piano composition.

First you need to draw out 8-bars on a piece of paper. I use 8-bars first because it's a relatively small space to "fill up" quickly. You don't have to use notation paper. Any paper will do. In fact, I use a spiral bound notebook with blank pages. I just write out 8-bars and voila, I've jotted down what will become a section of music.

Now, here's the interesting part. Most composers start with the melody line first. Nothing wrong with this. But if you really want to zap out a section quickly, start with the chord changes. Why? Because you can block out bars of music faster. Here's what I mean.

Say you want to create something in the Key of F Major. Great. Now we know that we have at least 6 chords to work with. By using just 3 chords, we can block out our 8-bars. How? Look Say we have the F Major 7 chord for the first 4 bars, then comes B flat Major for 2-bars and C 7 for the last 2-bars. We have now created a chord progression and charted it out. You can do this in under a minute. I swear it! It's that easy. Now all you have to do is decide upon the kind of arrangement you'll create for these chords.

It might be arpeggios, block chords, open position chords whatever. The point here is that by using chords, you can map out a harmonic territory. Now you can either create a melody using these chords, or keep it entirely textural. It's up to you! Try it.