Unfortunately there are so many ways to do this.
A pretty secure way would look something like this.
- Create a few paperwallets and store them at different safe places for example at your parents / bank vault / etc.
- Create an encrypted container on an USB stick and store the wallet file .wlt and the address book there
- When you need to do something NEM / crypto related then do it on a virtual linux machine
- The virtual machine file should also be located within an encrypted container file located on an USB stick
- Use as password manager like KeePass to store your passwords
- For everything else use a regular PC / laptop
Well it might be an overkill … it’s just a suggestion.
Keep in mind NEM will evolve and it’s value will rise which means you should have a tight security plan on how to handle your wallet private key passwords etc.
Maybe others will join and suggest something else / better / more efficient.
p.s. the above is not perfect but a start