I have bought a new A string for my viola, and it comes with two plastic/rubber pieces - a small sleeve that fits on the string, and a thicker circle of rubber with a whole in. What are these for? Does the sleeve sit at the nut? I think the other thing is to protect the bridge, but cant for the life of me work out how to fit it.

Please help!

The plastic sleeve is used to protect the bridge from the string cutting into it. I've known some people use the little rubber grommit instead, but they tend to damp the vibrations too much. Make sure only a small part of the sleeve is on the bridge (ie. most of it on the tailpiece side), otherwise that too will damp the vibrations.

Ask your teacher, or a luthier if you are still not sure about string fitting. Ensure the string is wound evenly around the peg too - don't wind the string repeatedly over itself.