Sometimes the work has been done for you, as in the example below. This concert band chart offers alternate 'World Parts' and they are free to download from www.alfred.com/worldparts
If there are no 'World Parts', there are still plenty of options for giving a brass band student a part without having to do any work. Highlighted on the score below are some of the alternative parts.
You can usually find a part that a brass band student is able to play without having to re-write parts. However, if you have a particular part in mind, you will need to transpose: for example, perhaps you'd like a tenor horn player to play the French horn part. In this instance you'll need to transpose the music up a tone.
Or perhaps you would like a brass band trombone player to play a trombone part. You will need to re-write the part from concert pitch bass clef into treble clef, transposing in B flat - the music will essentially look just like trumpet music.
The trombone is usually written in concert pitch, bass clef, however in a brass band it is transposed up a tone, and is in treble clef. It will look like trumpet music.