Set only the bit in the register that corresponds to the LED you want to light (i.e. set a 1 in any single bit in the register) and then send the byte to the shift register. The LED corresponding to the single bit set (or reset if 7 leds go on and one goes off 
) is the one you can control. Setting that particular bit to 1 or 0 turns that one led on or off.
Peter