Firstly a publisher/client approaches me with an idea and in most cases it's very specific and they can either provide a layout in the form of a PDF with the text already in place or certainly a place for it.
I then get started on thumbnails and larger sketches (involving sometimes much blood, sweat and tears if ideas just fail to come) until they culminate into a rough that I feel is good enough and interesting enough to show the client. Like this:
Then the client likes to edit the rough, maybe suggest additions or if there is a character, maybe tweaking it or completely changing it even! Eventually after a few amended roughs there is one they are happy with that get's 'signed off'.
Then of course you get on with the finished artwork, like this: