What MCU are you using? Dallas? Other? That makes a big difference on exactly how you do it.

But generally speaking, you configure one of your timers to generate an interrupt every X milliseconds. The timer interrupt takes care of resetting the watchdog timer.

See http://www.8052.com/320dog.htm here on 8052.com for a tutorial on the Dallas 80C320 watchdog timer.

Good luck,
Craig Steiner