There are books on mock codes, usually you can find one in your local
health library (at a medical school or academic/teaching hospital).
Here's one on Amazon:
These books give you the cases and help give information about what
information to give and when.
How you run a mock code depends on who's participating: physicians,
EMTs, nurses, nurse practitioners, students, etc. Everyone should be
able to assess the ABC's (airway, breathing, circulation) and start
CPR as necessary, but their individual skill level will determine how
much you do beyond that. Generally, the person running the session
will say something like "a elderly man collapses on the floor" and
want to know what you would do, walking the group through the
assessment, stabilization, treatment and diagnosis.
If you have a specific question after looking over one of these case
books, please post it.
Hope this helps get you started.