I have experiences with doing this, so I'll lend you my tips, but they might be different than the way than Piano Squall does it... In that case, I'd be pretty interested myself on how he actually does it!!
1. What I do first usually, is to be able to sound out the melody and be able to play it by ear. (for example, the first phrase-- I don't like writing the entire piece at once-- it's boring to me). After accomplishing this, you will be able to play the melody line. You want to make it as simple as possible, just plain rythm and notes (not even chords).
2. The next section is probably the harder part. After sounding out the melody, you want to listen to the underlying chord. This is not easy sometimes, but "most" of the time, the first chord will start on the tonic. But how do you find the tonic? When you doodled out your melody with your right hand, you must have noticed that sometimes, you played some black keys along the way. Knowing which black keys you played during this melody will help you determine the tonic:
For example, in DNAngel opening:
A G# F# C# G# G# E F# A G# E C#
you noticed that the sharps played in this opening is F# C# G#. Which scale do you know has these three sharps? You would first think of the Key of A, however, the tone quality of this song is sad, so it must be the minor of this key (which is 3 half steps down from A which is F#) Thus I know the opening of the song starts with the chord of F#.
So from this, you can conclude that you have:
A G# F# C# G# G# E F# A G# E C#
F# E D E A
This is the basic melody and the basic bass line. This is the barebones of the song, and from here, you can expand it. One way of expanding it is to
1. Expand the left hand
There are many ways of doing this, arpegiating it is the easiest way out, though it can be boring unless you start to vary it a bit You can do stuff like 1-3-5, or 1-5-8 or 1-5-8-10 go backwards fowards, continually go up, play the plain chord, invert the chords, the possibilites are endless and you have to chose them wisely to give the music you right the texture you want. For example, on Piano's Squall's Version of Evangelion opening, he opens the piece with just plain chords (which is awesome because it fits so well). If you tried to apregiate it, it becomes sloppy and overdone.
If you added the expanding of the left hand with the skeleton above, you would have a plain melody(right) and nice harmony(left) to go with it.
2. Expand the right hand
The left hand notes you are playing are usually the root of the chords in which you're playing from here, you can pick the 1st 3rd or 5th degree of the chord and tack it onto the BOTTOM of the right hand notes. (You can basically put them in anyway you like-- they would sound good almost at all costs, sometimes I just randomly add them to whatever I feel like at the moment but you can never go wrong as long as you play the 1 3 or 5th degree of your left hand note.
At this point, you piece would be done!! I hope I was helpful, I hope mike reads this so he critique my method to see if I can improve as well XD
