Transposing Individual Parts in a Score

Hello everyone, How do I transpose an individual part up or down an octave in a score? Let's say I have a piece for 4 flutes but I want to transpose one of the parts an octave lower for viola. Of course I will change the instrument to viola, but I want the whole part an octave lower. What should I do? Remember, I only want one of the parts to be an octave lower.