It is quite easy. Just the same way you keep your school certificate and other important documents out of reach of others, that's how you should keep your phrase.
The difference between a school certificate and a seed phrase is that if your certificate burns during a fire, you can get a duplicate. But if your piece of paper with the seed phrase burns in a fire, then no one can provide you with a copy of it. So a place inaccessible to others cannot guarantee the safety of saving the seed phrase.