Depending on what interrupt you want to do, you need to

1.) Set a vector to your interrupt code in the interrupt vector table (depends on your processor variant)

2.) Write your interrupt code. Make sure the code is exited with RETI instructions NOT NOT RET

3.) Make sure said code can reset the original interrupt flag if you need to.

4.) Make sure that the code leaves no traces of itself when you exit from it. So save the PSW and and any other registers you might bugger up.

5.) Enable the interrupt in your main program..

Make sure that the interrupt is not repeatedly fired by accident, easily doen with INT0 and INT1 in level activated mode.

Steve