Actually, it’s quite simple, as Donna Spencer points out:

Just make it up.

Wait? Is it that simple? But in order to make it up you just ned one thing: information.

You need to understand what you are trying to achieve, what users of the service need and know, and you need to know the content well. If you don’t have these things, it will be hard. But if you do have them, pulling them together into a first draft is surprisingly easy.