Obtain sheet music for the piece you wish to arrange (get a hard copy or transcribe it aurally).
Analyze the music, identifying the melody and the harmonic progression.
Note additional elements, such as counter-melodies, dynamics, important rhythms and tempo changes.
List the instruments you need parts for. Marching bands usually consist of woodwinds, brass and percussion instruments.
Outline the melodic progression. Plan: Which instruments will have the melody? At what point? Where will it move next?
Outline the harmonic progression. Plan: Which instruments will have the harmony? At what point? Where will it move next?
Set up the score on staff paper or a computer music program (such as Finale or Sibelius).
Notate the key signature and time signature.
Write out the melody.
Write out the harmony.
Add dynamics and phrasing.
Scan the piece, looking for: transposition errors, melodic errors, harmonic errors, rhythmic errors and "holes" (parts you forgot to write).
Save and/or make copies of the music manuscript.
Write out individual parts based off your score and distribute them to your musicians.
Rehearse and make any corrections necessary. Have fun!