How to secure my keys?
Imagine the blockchain is a padlock and your private/secret key or mnemonic seed is the key to gain access to the balance in your wallet. It is highly recommended to backup your key/seed to someplace only you can access it. For example on a text file inside a password-protected ZIP/RAR archive, stored on multiple (preferably encrypted) USB drives. Secondly, since your offline wallet stores your keys as well, it would be wise to consider running it in a secure environment such as a VM and/or air-gapped laptop.
Below are some popular methods of protecting your keys. As a general rule, don't over-complicate things and avoid single points of failure. For example, forgetting your password or storing your keys on a single USB drive with the potential of loss or failure. In such cases, you may lose access to your funds without any means for retrieval.