I memorize it. A 12-word seed is a lot easier to memorize than most people thought. But I understand that this is definitely not for everyone. another way is writing it on a paper. with a paper, you can just keep it at your house or in your physical wallet. I'm keeping my phrases written on 2 pieces of paper. One of them is among my notes but it's hard to notice. Somebody would have to know what to look for to find it.