The five threats, named
Theft of the backup. Destruction by fire or water. Loss through moving house or forgetting the location. Inheritance — nobody else knows it exists. And degradation, where ink fades or a cheap medium fails.
A good plan has an explicit answer for each. If you cannot say what happens in each case, the plan is one scenario wide.
Media that actually lasts
Paper in a sealed container is fine indoors and bad in a flood. Stamped or engraved metal survives fire and water and is the standard answer for meaningful amounts. Both are vulnerable to someone finding them.
What does not work: a photo in a cloud gallery, a note in a password manager you also use for email, an encrypted file whose password you will not remember in five years, or a text message to yourself.
Geographic separation without duplication risk
Two copies in two locations halves the chance of destruction and doubles the surfaces available to a thief. Splitting the phrase across locations sounds clever and usually produces an unrecoverable backup, because partial phrases are far less useful than people assume.
A more reliable approach for larger amounts is a multisig with geographically separated signers, which is designed for exactly this problem.
Testing and succession
An untested backup is a hypothesis. Restore it into a fresh wallet application once, confirm the first address matches, then wipe that installation. Doing this when nothing is wrong is far better than doing it for the first time in an emergency.
Decide what happens if you are not around. A sealed instruction with a trusted person, or a legal arrangement, is the difference between an inheritance and a permanently frozen balance.