The general trick has to do with how the program's code and variables are layed out in memory. For example, when a function is called the program (code inserted by the compiler) must store the address of the instruction to return to. So if this is the 32 bit word just before the beginning of the stack, one could do:

 void foo()
 {
    int array[5];
    int var = 0;
    int var2 = 0;

    // read in user input
    printf("Enter index and value to write:");
    scanf("%i", var);
    scanf("%i", var2);

    // malicious user might set var to -1 and var2 to an address to execute
    // if say the 32-bit value before the stack variables is the instruction to
    // return to
    array[var] = var2

    // return now goes to malicious code
 }

(So your job is to construct code so that such a thing is not possible. :) )

The rules for how a function call is implemented, stack variables allocated, values passed, and return values returned back is called the calling convention. I reccomend reading the attached article for a good indepth coverage of C calling conventionts.