This program is a simple modification of the
program to control the on-board LED:
just change the value of the pin associated with the LED being
controlled:

/* Blink an external LED attached to pin 8 */

# define LED 8

void setup (void) {
    pinMode(LED, OUTPUT);    
    digitalWrite(LED, LOW);
}

void loop (void) {
    digitalWrite(LED, HIGH);   // Turn the LED on.
    delay(1000);               // Leave it on for 1000 ms, which is 1s.
    digitalWrite(LED, LOW);    // Turn the LED off.
    delay(1000);               // Leave it off for 1000 ms.
}