I went rummaging through the internet and found the following which seems to me a good overview of the process.
"Orchestration
Now that we are familiar with the different instruments of the orchestra and have a good idea of how to incorporate them into a cohesive ensemble, we begin our life long journey of discovering how to artfully extract the most possible beauty from this collection of versatile sound producing devices. With so many choices of instruments, the task can seem a little daunting at first, but through practice, we can recognize patterns that will help guide our decisions and yield powerful and provocative orchestral textures.
Before we begin assigning parts to instruments, it is very important that your music makes sense to the ear in an absolute sense. What I mean by absolute sense is that if you were to have one or two pianists play the entire composition, would the music be easily digested by the ear with clearly defined musical lines or would it be difficult to discern the different parts from one another? If the answer is the latter, you probably want to revisit the composition of the music before you attempt to orchestrate it. Well composed music will always translate better to the orchestra, so take the time and make sure you're happy with the notes before you start deciding which instruments should play them.
Balance
Balance is arguably the most important factor in successfully orchestrating a piece of music. During orchestra rehearsals, a large percentage of time is spent on adjusting the dynamic levels of individual instrument parts to assure that all parts are audible and an even, balanced texture is achieved. Our goal as orchestrators is to minimize the need for these dynamic adjustments so that every instrument is clearly audible and free of conflicts between instruments with little effort by the performers. The end result will have a greater sense of cohesion, rather than sounding like a pack of misfits all competing to be heard.
The different factors that affect balance are as follows:
1.Number of instruments playing the line. The more players that are playing a given line, the louder and more powerful the line becomes. That being said, equal numbers of instruments of equal weight in tone will produce an even balance. For example, two flutes playing a melody line in unison will balance evenly with two bassoons playing a bass line in unison.
2.Instrument's family. As a general statement, brass and percussion instruments are the strongest members of the orchestra, then the strings, and then the woodwinds. Careful consideration must be taken when balancing parts among the different families. It is often best to balance each family within itself to achieve an even balance in the whole orchestra.
3.Instrument's register. The quality of tone and degree of weight varies greatly depending on the instrument's register. It is very important to understand these differences to properly balance textures. Refer to the range charts given in the "Overview of Orchestral Groups"
4.Vertical relationship to the other parts. The top line of a texture is the easiest to hear, the bottom line is the next easiest, and the middle voices are the most difficult.
5.Space between parts. The farther apart voices are from each other, the easier it is to discern them from one another.
6.Degree of movement. The more a part moves, the more it sticks out within a texture. The less a part moves, the more it fades into the background.
7.Dynamics. Although, balance through instrumentation is commonly the best approach, certain instances call for instruments to play at different dynamic levels. Suppose you want to have your first violinist stand up and take a solo. You would likely want to have the orchestra playing two dynamic shades lower than the first violinist so as not to drown him/her out.
Blending
One of the most interesting aspects of orchestration is the blending of timbres. Just as an artist mixes paints to produce different shades of color, an orchestrator blends timbres to produce different shades of sound. With the sheer number of instrument and articulation combinations, the possibilities for different timbres are virtually infinite. As an orchestrator, it is your job to experiment with all these different combinations and discover the ones that best suit the music you are orchestrating.
It is important to understand that the more instruments you have playing a given line, the less colorful the line becomes, but with loss of color comes gain in power. As you begin blending timbres, there are a few different factors that tend to produce better results:
1.When the instruments are in the same family and even more so in the same subgroup (e.g. single-reeds).
2.When the instruments are evenly balanced with each other (e.g. 1 trumpet playing f = 2 french horns playing f).
3.When the instruments play adjacent voices.
4.When the instruments play parts that are similar to each other, especially in articulation.
5.When none of the timbres attract more attention than the others.
6.When the intervals between the instruments remain relatively consistent.
Function
A major part of making decisions when it comes to balance and blending is function. It is important to understand how a given line functions in relation to all the other parts. For this purpose, we can break it down into three different functions from greatest to least importance: melody, bass, and accompaniment. The following methods can be used to help maintain a clear function throughout your arrangement:
•Melody◦Keep line on top of the texture
◦Double on the unison or in octaves
◦Use instruments with greater strength
◦Use the more powerful registers
◦Use louder dynamic markings
•Bass◦Keep line on the bottom of the texture
◦Double on the unison or in octaves
◦Use instruments with powerful low registers
◦Use louder dynamic markings
•Accompaniment◦Keep lines in the middle of the texture
◦Use less doubling
◦Use instruments with lesser strength
◦Use the weaker registers
◦Use instruments of similar timbres
◦Use softer dynamic markings
Closing Thoughts
Although there are many different techniques to achieve favorable results when it comes to orchestration, there are no rules set in stone. Sometimes the best moments are those when the rules are broken. For example, the opening of Stravinsky's "The Rite of Spring" is a bassoon solo played at the very top of the bassoon's range that has a plaintive, almost piercing quality to it. Most orchestrators would never dare give that part to a bassoon, especially not at the very beginning of the ballet. But that is what makes the music so great and innovative is that he took chances and broke the rules, and it paid off. At the premiere, the music had such a profound effect on the audience that a riot broke out in the concert hall!
Some other things you can do to become a better orchestrator are:
•Learn as much as possible about each instrument. The more you know about each of the different instruments, the more comfortable you will feel giving parts to them. Get to know some orchestral musicians. They know more about their instruments than anyone else and can provide you with a wealth of invaluable knowledge.
•Read books on orchestration. The purpose of this article is to be a crash course in orchestration and was by no means meant to cover all the topics associated with orchestration. Go to your local library and check out some books on orchestration.
•Practice and experiment. There is no substitute for experience, so don't be afraid to try new things. Even if the results are not as favorable as you had hoped, you're learning. Over time you'll develop an arsenal of techniques that you can use to achieve any hue of expression you desire.
Stravinsky has been quoted for coining the phrase, "Good composers borrow, great composers steal." Even if you try your hardest to sound exactly like someone else, you'll only end up sounding like yourself trying to sound like that person. No matter what you do, you'll always sound like yourself, so why worry about it? Steal away! They've already done a lot of the hard work for you, so take advantage.
Furthermore, if you're going to steal, steal from the best. Why would you ever want to steal from an average composer? It's only going to make your music sound average. Steal from the best, and your music will reflect that.
Orchestration may seem intimidating at first, but the more you do it, the better you will get at it. The orchestra can be the most versatile medium you will ever use, so spend time to get comfortable with it. Leonard Bernstein said it best in his 1958 broadcast of his Young People's Concerts series, "The right music played by the right instruments at the right time in the right combination: that's good orchestration." "
Source:
http://www.gamedev.net/page/resources/_/creative/music-and-sound/a-brief-guide-to-orchestration-r2718