Get sheet music for the music you want to arrange for the string orchestra. You can either buy a hard copy or transcribe it aurally.
Analyze the music, making notes either in the sheet music or on a separate written outline. Identify the melody and the basic harmonic structure.
Note additional elements after you have the basic structure outlined. Counter-melodies, dynamics, rhythmic figures, tempo changes and instrument or vocal changes all add important aspects to the music.
List the instruments you need parts for. String orchestras include first and second violins, violas, violincellos and string or upright basses. Guitar, piano, percussion, vocals and other instruments may be used sparingly.
Outline the melodic progression. Plan which instruments will have the melody and at what point. Know where it will move next and how many voices the melody will be played in at any point.
Outline the harmonic progression. Plan which instruments will have the harmony and at what point. Know where it will move next and how many instruments should be playing the harmony at any given time.
Set up the score on staff paper or using a computer music program (such as Finale or Sibelius).
Notate the key signature and time signature.
Write out the melody, then the harmony, and, finally, add dynamics and phrasing markings.
Play the music back to yourself via the computer program. Scan it while you go, looking and listening for transposition errors, melodic errors, harmonic errors, rhythmic errors and "holes" (parts you forgot to write or parts where the texture is weak).
Save the music manuscript and print copies.
Make copies of each part to distribute to your musicians. Rehearse with a pencil in hand to make any necessary corrections.