Step by step
Write it on paper, by hand
Number the words 1 to 9 and copy them exactly. Check the spelling against the screen.
Do not take a screenshot
Screenshots go to your photo library, and from there to iCloud Photos and every device you own.
Do not store it digitally
No email drafts, no notes app, no chat with yourself. If you use a password manager you fully trust, that is a reasonable exception.
Put the paper somewhere durable
With your passport, in a safe, or in a sealed envelope with other important documents. Not in your phone case.
Consider a second copy
A copy in a second location protects against fire or loss. Only do this if both places are private.
Test it once
On the lock screen, tap Use recovery phrase and enter the words. Knowing it works is worth the two minutes.
If the phrase may have been seen
Open the vault, go to Vault Settings and choose Change Recovery Phrase. The old phrase stops working immediately.
One phrase per vault
If you use several vaults, each has its own phrase. Label the papers in a way that means something only to you.