An example of fork() and wait() functions
 

Please tell me how to use fork() and wait() functions to create a child process and transform that process into a zombie state.

Take a look at here:

http://www.cs.cf.ac.uk/Dave/C/node22...00000000000000

The link above tells you how to fork() a process.
To create 'zombies' I believe you have to use
signal handlers.
When a child process terminates it send SIGCHLD signal
to its parent process .
If the parent process *ignores* the SIGCHLD signal
the child becomes a zombie.

To transform the child process into a zombie you just have it call exit(), and have the parent not pay attention to the SIGCHLD signal as pankaj99 said.

I use waitpid(-1, NULL, WNOHANG); in the parent process to clean up any zombie children, since in the program that uses it I don't care when or why the child exited.