Understand how the chromatic scale works. The chromatic scale consists of 12 notes between octaves. Starting on C, the chromatic scale is C, C#/Db, D, D#/Eb, E, F, F#/Gb, G, G#/Ab, A, A#/Bb, B. Some of the notes have enharmonics that can be spelled more than one way.
Identify the trumpet notes that need to be transposed to trombone notes.
Transpose the trumpet notes down a major second. A major second is two steps in the chromatic scale. For instance, if you have an F in the trumpet part, you will play an Eb since Eb is the second step from F.
Transpose the notes down another octave and write them in the bass clef so the trombone is able to play the part comfortably in its range. An octave is the distance from one letter name to the next. For instance, an octave above middle C would be a C on the third space of the staff in treble clef.