Find the total range of pitches in your composition. Look for the following elements: melody, countermelody, harmony and rhythmic elements. When looking through possible orchestrations for a template, look to see if each of these elements fits the range for each of these components.
Search through orchestrations that fit your style of music. If your music is clear with a solid melody and chord structure, look toward Mozart's symphonies. Music that has a slightly darker texture with more counterpoint should rely on Beethoven's orchestrations. Music with several easily definable countermelodies, riffs and gestures should look to the Mahler Symphonies. Finally, music that is contrapuntal and complex should look toward Bach as a template.
Copy each component into the same range as your orchestration to create the same balance and texture. The melody should use the same range and instruments as the orchestration you are using as a template. The harmony should be in the same range and use the same instruments as the template as well. Each component should match up to get a balanced orchestration.