Back it up in a safe place such as a safe and keep it protected from anything related to internet access. I wrote it in a notebook then laminated it well, made a QR code and made it like a card. Apart from that, the most important thing is to entrust a trusted person like my wife to have access if something untoward happens to me. Meanwhile, what my friend did was save it in a safe deposit box at the bank and appoint a lawyer to keep it in the hands of the heirs, but it requires tax payments every year. Everyone has their own way which they think is safe, currently I personally think my way is quite safe but it doesn't apply to other people. Checking to make sure assets are still safe, I open the safe and check it every 6 months.