Many people do not save their mnemonic phrases securely. The safest way to store them is not about where you keep them. I recommend a storage method: do not save the mnemonic phrase directly; instead, convert it into a set of passwords that only you know. For example, take the letters 'abc' and assign a=1, b=2, c=3. Store these numbers in a secure place. You know what these numbers represent in terms of the corresponding letters, but others do not. In case of a leak, others will not know what these numbers mean.