a mock up is simply the way in which or the possibility of how a design is going to look once finished. It may or may not change.
When doing a mock up you go based on what the client wants for his or her website. Try to stay away from "editing" other persons work as it is not truly yours. There is no harm in using a design as a creativity tool but once you begin editing another persons code to make it yours, it may lead to an obsession or a simply a lack of creativity on your part and would not help your career.
The client would have set features they want in their website plus you should always try to get the client to list a few good websites they like and try to find out why do they like them. This will make your designing process much easier as it allows you as the designer to see similarities in a design (if you have a trained eye of course)
A mock up does not need much detail. Simply start off simple with the guideline notes you took from the client and you should be find. You could seek to improve on the design later down when the design starts to develop a bit (by more questioning of the client)
Hope this helps and good luck