It's a matter of how you're explaining your problem, or how far you've gone on your own to figure it out. How to ask questions the smart way.
Even just posing the right question would walk you through the problem and you might answer your own question before hitting the Post button.
"Hi, I want to control multiple 74HC595 packages from serial input. Kind of like this scheme on this blog [link]. Over serial, the PC will send "A255" to indicate the first package (A) gets eight high bits (255 or 0xFF)." Pick one:
"I can send bits and bytes to a 74HC595 okay, but I can't figure out how to parse the serial input." or "I can parse okay, but I have no idea how to turn an 'A' into a specific 74HC595 package." or "I can parse and select packages okay, but I have no idea how to latch the bits to the selected package."