How do I send off a handwritten letter to a YouTuber?

If you know the person, you could ask them for their mailing address. If it is someone whom you don't know personally but to whom you want to send a handwritten letter, you could try searching on the internet for their name (if their real name is known) or checking to see if there is any other information online that might list a mailing address (such as a P.O. box) for the YouTuber. Once you have found the person's mailing address, you will write the letter and then fold it up and put it in a mailing envelope (you can even buy one at the post office if you don't have one) and then put a postage stamp on it. You can purchase one stamp at any post office if you don't already have some. Then the person working at the post office can mail it for you or can stick the letter in any post office box.