Listen to the song repeatedly. You should be able to sing along with all parts of the song, including the melody, bass lines and backgrounds. Make mental notes of what instruments would be best suited to play each part.
Create an outline of the arrangement. Include the total number of bars and be sure to mark measure numbers where themes, backgrounds and other important figures occur. Arrangements for marching band need to fit specific time constraints, so be sure the piece is not too long. The outline will make your arranging faster and more organized.
Notate a sketch of the arrangement. This should include the lead lines for all introductions, melodies, backgrounds and tags. Be sure to use logical instrument choices for each voice. This may require some creativity if the original instrumentation was for an ensemble somewhat unrelated, such as a rock band. Be sure that you do not exceed the normal range for any instrument. It may also be necessary to simplify some rhythms for ease of reading.
Finish the arrangement by harmonizing each line and completing any transitions that need to occur. Check the range of each instrument again, as it can be easy to overlook a note that is too low after harmonization.
Check that your chords, voicings, accidentals and transpositions are accurate. If using music notation software, this can be accomplished by scanning the score while using the MIDI playback.
Copy each individual instrument part and make adequate copies. If using music notation software, you can choose to print all parts from the score.