A.K.A
In fact, arrangement is an exercise and art form of its own. In classical circles, the arranger is usually referred to as the orchestrator. In dance and popular music, it's called remixing. Jazz, big band and in formal band situations, they are referred to as arranger. Whatever they may be called, arrangement is not to be taken lightly. It is another skill that must be developed and cultivated. Some of the best producers have a knack for putting together great arrangements. 'Q' (Quincy Jones) immediately springs to mind.
Decisions, Decisions
How do you go about honing your arrangement skills? Like everything else, practice makes perfect. It's a matter of taking a song, piece or even a simple melody and creating different arrangements. They could be all in the same genre with different instrumentation (think string quartet verses symphony, or standard rock version verses acoustic version), different styles within a genre (pop-ballad verses dance/pop crossover, or punk verses alternative), or different genres altogether. You must decide on what you're going to do from the outset. Are you going to make some orchestrations based on a simple theme or are you going to turn a country song into a dance-floor hit? Arranging in the different styles and genres is an entire book in and of itself. Without getting into genres and specifics, lets look at some general things that you should be thinking about when starting a new arrangement:
1. Have a goal. I know you've heard this one a million times before but it really counts here. If you have a pop song, what are you going to arrange it in to? You must have a clear idea before you start because your goal will dictate a lot of your decisions. If you're going to make that pop song into a dance remix, you're going to make completely different decisions than if you were going to turn it into a ballad.
2. Decide on instrumentation. As soon as you figure out what kind of arrangement you're going to write, you're going to have to make a decision on the instrumentation. You might be thinking that this may be limiting you but in fact it's the opposite. By deciding on a set group of instruments, it makes you more creative in trying to stick with that group. Having too many choices in this area may be more of a deterrent to your creativity than an asset. Also, sticking with a set group of instruments say a basic drums, bass, guitar, makes it easier to stick with the genre that you've decided on from the outset.
3. Decide on tempo, feel. When you decide on what style your new arrangement is going to be, immediately there will be an inherent range of tempos that will suite the arrangement. Keep in mind that when deciding on the genre, it's also implied the feel of the song within that genre. For example, you don't just decide to do a jazz version but a jazz-ballad version. This automatically denotes a range of BPM that would be suitable for your arrangement. The same goes for dance, hip-hop, etc. For example if it was a hip-hop remix, a BPM above 130 would be unusual; the same goes for a dance remix below 100 BPM.
4. Decide on form. This is another decision that would be inherent in the style. For example, if you were doing a dance remix, the build and release of a dance song would be paramount to making your arrangement work. If you were doing a jazz arrangement, you would follow the AABA form and add a chorus for a solo. If it was a pop song, you would stick to the general forms using verse, chorus and bridge. There are tons written about form in the various genres. Classical music for example has tons of various forms that are essential to learn when composing for orchestra. While there are many variations of forms and many ways form can be manipulated, it's essential to know what you're doing and the reason why.
Let's take a look at the areas that you have to consider when working on your arrangement:
Rhythm/Groove
This one may seem obvious but there's always a lot of decisions that must be made here and they shouldn't be taken too lightly. The basic rhythms for each genre each have a set of rules and standards. Are the rhythms in strict time (like techno and club), a little more loose (like some alternative and folk), or more rubato (like classical and traditional folk music)? Some genres swing, some don't; the answer isn't always obvious. You might assume that swing is a jazz rhythm but it's used in other genres to varying effect. Blues also swings; as does rock, latin, pop and various forms of dance music. They all use it in different amounts and the application is different. Then there are the 16th note shuffle used in hip-hop and dance. There are different drum patterns and specific drum sounds for each genre. There are also different ways to phrase your melody based on the genre. Some genres play the melody straight where as others tend to make the melody more syncopated. The instrumentation in the rhythm section would be another deciding factor. Is it a basic drum kit, Latin percussion, or an 808?
Form
Learning forms in music is another area where you would want to sit down and take some notes. You may already know the forms of some popular music that you are familiar with, but you shouldn't stop there. When listening to other styles of music, take note of the form. How many sections are there? How long is each section? What's the typical order of the different sections? What's the general feel of the different sections? If you're unfamiliar with a genre of music, it may be hard to tell where some sections end and others start (as in some classical forms). Every genre of music has forms that it uses over and over again. If you're new to writing in a certain style, start with one of these forms and go from there.
Harmony
What is the general harmony used in that particular style? Is it your basic triads as in pop, country and dance? Is the harmony a bit more involved like some house, alternative, metal, and latin? Or is the harmony a huge factor as in jazz and classical music? Are there progressions and harmonies that used more than others? If there are, memorize and get to know those first. Even beyond the basic harmony, there are idiosyncrasies that are prevalent to each style. An RnB keyboard player would play different voicings than a traditional jazz player. Are you going to change the harmony? Make it more complicated? Simpler? Change the tonality (major, minor, modal)? There are dos and don'ts to each genre. As soon as you change the harmony for a song or piece, you may unwittingly move it into another genre. If you change the harmony enough, it may become a jazz tune instead of a pop song.
A New Skill Set
As you can see within each of these decisions is a skill set of its own. Each must be taken into consideration when putting together an arrangement. There are general rules for each genre and style of arrangement that you write. These rules are never written in stone, but it's good to know them. It helps keep your arrangement genuine to the genre. As you get better at remixing and re-arranging, you may find yourself breaking a lot of the rules. There are tons of books out there about arranging and remixing in various genres. There also tons of books on how to play the various styles on their perspective instruments. These are all useful when writing and arranging parts particular to the style. Pick up what you can, absorb and start writing.