What is piano arranging all about?
Arranging at the piano is the process by which you take a written piece of music and rework it, adding new bass accompaniment, fills, or even slightly altering the song's structure. And while it's a process that takes years to truly master, anyone with a basic education in piano and a working knowledge of a few key techniques can create an inventive, satisfying arrangement. It all boils down to one thing: chord recognition.
Throughout this course, you've learned to play piano using two fairly different methods: playing by written music and playing by chord symbol. Playing by written music is exactly what the phrase says it is -- playing the exact notation on a piece of sheet music. But playing by chord symbol is a little different. Instead of following the harmony note by note, you follow the chord symbols (i.e. C7 or F) written above the harmonies, filling in the gaps with...well, whatever you want as long as it sticks to those chords. Of course, you'll still read the melody (it is, after all, often what makes the song recognizable) but even that is completely open to interpretation. Playing by chord symbol allows you a freedom that playing by written music simply doesn't. The freedom to create. The freedom to invent. The freedom to arrange.
Does that mean playing by written music is less important than playing by chord symbol? Absolutely not! The ability to play by written music is an extremely valuable skill, one that even some of the most famous musicians don't possess. And while you don't necessarily need to know the skill backwards and forwards to create great arrangements, it's a tremendous help.
Think about it this way. Some of the most revered modern artists create paintings that look very simple, very rudimentary. But the majority of those artists went to art school for years before they began creating that sort of work. They learned the fundamentals of drawing and painting, of color composition and light; they learned to draw or paint something exactly as it actually looks. Only after they mastered those skills did they move on to create the simple, yet often innovative, work that hangs in galleries and museums -- work that still abides by several basic principles. They learned the craft before bringing their imagination into it; after all, you have to understand the rules in order to break them.
The arrangement you create is bound only by the limits of your imagination. Try everything and don't get discouraged -- you'll get the hang of it eventually. Now pick a song and get to it!