Imagine I'm the head of some federal agency, let's say the Treasury. The president Tweets that he just signed a bill into law that requires me to send $600 checks to all Americans. How do I officially know that the president signed it and that I need to boot up the (metaphorical) check printing machine? Do they fax me a copy of the signed bill? Does GPO publish an official notice? Do I call the White House Chief of Staff? It's a super basic question but I'm having trouble finding the answer. This question comes close but it's slightly different.
They are published by the Government Publishing Office and stored in the Library of Congress. So if you need to check, you know where to look. You, as head of a government agency, are expected to keep abreast of legislation that affects you (and you have a whole agency to help you do this). That is your job, and you get fired if you don't do it.
Surely it will be discussed in Cabinet meetings (so you will know in advance of the President's intentions) and the President might contact you to let you know as a personal favour. Your agency will get a copy of the law and be going through it to give you the best advice on exactly how to proceed and you will have developed a plan for what to do. But ultimately it is your job to be aware of the law as it affects you.
This is the same as any other law. If a State passes a law that bans smoking in church, I (as a regular citizen who enjoys a pipe of tobacco as I pray) don't get faxed notification. I am expected to keep abreast of the laws that affect me and can't claim ignorance as a defence. Of course, that is criminal law, but the principle is the same. It is the responsibility of each person to obey the law, and for a government officer, that means following laws that instruct them to carry out particular plans. Failure or refusal to do so is incompetence or subordination and could result in immediate removal from post.