Reddit reviews Preparatory Exercises in Score Reading (Vorbereitende Ubungen im Partiturlesen)

I'd recommend starting with this book. It'll ease you through 2, then 3, then 4 voices, and introduce new clefs one at a time.

Set up a metronome. At a very slow tempo. Play one part and read another. Do this every day, several times a day.

Oh, and ask your professor if you can sing on English letter names rather than fixed do. They're functionally and semiotically equivalent, but easier for English-speaking musicians to process.

Are you taking a conducting class? When I was taking conducting classes, we had to read these transposed instruments on the piano. It was tough, but definitely worth it since you can pick up a score and start playing it on the piano.

I used this book to study: Preparatory Exercises in Score Reading (Vorbereitende Ubungen im Partiturlesen)