Play through the alto line of choir music on the piano or electronic keyboard. As you are playing, listen carefully to how it sounds. Mark any parts you think may be difficult with a pencil, in case you need to come back to these later.
Play the alto part to the choir music on the keyboard while singing along. If this is difficult to do, try playing and singing more slowly so you can find and perform all the notes correctly. Repeat this step until you perform that part correctly three times in a row. If you find yourself making errors in the same place every time, mark this area with a pencil and repeat step one with this area until you feel confident that you understand how it sounds.
Play your starting note on the piano. Sing your alto part without playing the piano along, but stop yourself at random parts and check your note on the piano. If you find that you are not on-pitch with the piano, go back to step two and keep practicing your part with the piano playing until you are confident about it. This may take many repetitions.
Play your starting note, and sing your alto part a capella (without the piano). Check your final note. If you are not on-pitch, go back to steps two and three and repeat until you feel confident to give a capella singing another try. If you are on-pitch, repeat step four until you can sing the part and remain on-pitch three times in a row or more.
Get permission to record a rehearsal from your choir director. At your next rehearsal, record all the parts where the director is teaching the alto part.
Practice your part at home by listening to and singing along with the recorded sections of rehearsal.
Record another rehearsal (with permission from the director) once you have mastered the first recording.