The trumpet is a transposing instrument that sounds a major second higher than written. If a trumpet plays a Bb, it will sound like a C. Transposition is the process of moving notes up or down in pitch by a constant interval. Changing the trumpet notation allows the majority of the notes to stay within the musical staff. Although transpositions make it easier to read trumpet music, this does make it difficult to play trumpet music on nontransposing instruments such as the trombone.
Learn the notes of the chromatic scale. The chromatic scale consists of C, C#/Db, D, D#/Eb, E, F, F#/Gb, G, A, A#/Bb, B.
Learn how to write a major second. A major second is both two half-steps and one note name away from the starting pitch. Using the chromatic scale, D# is both two half-steps and one note name away from C#. While Db produces the same sound as C#, it is technically incorrect to transpose to a Db.
Transpose all of the trumpet notes up a major second and then down an octave. An octave is simply one note name lower or 12 half steps. The trombone has the same basic range as the trumpet, but it plays one octave lower.
Write the new part out on a sheet of staff paper using the bass clef instead of treble clef. The bass clef is the clef used for instruments that play the majority of their range below middle C. Treble clef is used for instruments that play primarily above middle C.
Download a notation program such as MuseScore to input the trumpet part and then automatically transpose the notes.
Take your time writing the parts. Even a single wrong note will ruin the performance.