Ask HN: Where can I find an example of a professional functional specification?

I'm a freelance developer and I have managed to land a very large contract (6 figures). It's an ecommerce web project (design and development) and I'm very keen on making an excellent impression.

I promised the client that I would draw up a full specification of the work that I will be providing - however I'm not a very confident writer and I don't know where to start.

I've read Joel's guide to functional specifications and had a go based on his example yet I can't help but feel that my attempt is amateurish and embarrassing.

Can anybody point me in the direction of a professional specification that I can use for inspiration or give me some pointers - What exactly is expected from a person or company when a client agrees to spend big money on a project.

Thank you.