Hardware Wallet Seed Safety Rules

In general, you should never store or process your seed on a computer or any mobile device. Just never ever! End of story.
Unfortunately, calculating an SHA-256 hash by hand is practically impossible without a computer. If you decide to do it, you must be extremely careful and strictly follow the safety rules!
You should always assume the worst-case scenario while handling your seed. This means assuming that someone is actively trying to trick you and steal it. You must keep this fact in mind at every single step! This applies whether you are dealing with your best friend, the ledgernano.tips website, or even software made by Microsoft.
To exclude the possibility of stealing your seed, you should always operate on an offline computer without the Internet (and any other network) connection. If you didn't write the application code, you never truly know how it works. You never know if the application will not send your seed to someone else. For the same reason you should never put your seed to any website calculator. This would be insane!
But staying offline is not enough!
You never know if the application will not try to store your seed and send it to someone as soon as your system restores the Internet connection in the future. In the worst case scenario you should always assume this. Furthermore, even if the application is 100% secure (you wrote your own code), you still never know if your operating system is free of spying malware. And if so, then the consequence is exactly the same.
To avoid such a risk and stay secure, you should always follow these steps:
  1. Copy the application to the computer
  2. Disconnect the Internet (and any other possible network!)
  3. Put your 23 seed words and calculate the checksum
  4. Erase all data from the HDD, and delete the calculating application, including emptying the system Recycle Bin (Trash Bin)
  5. Restore operating system to the factory settings (eliminates the risk of any spying malware existence in your system including the calculating application)
  6. Connect the Internet
Restoring system to the factory settings is very inconvenient and implies a lot of future work because you need to install all your software again. But only such a manner guarantees you safety. If you want to avoid restoring system to the factory setting and you are absolutely sure that your system is free of malware, then you can follow these steps:
  1. Create the system restore point
  2. Copy the application to the computer
  3. Disconnect the Internet
  4. Put your 23 seed words and calculate the checksum
  5. Erase all the data from HDD, erase the application including system Recycle Bin (Trash Bin)
  6. Restore system to the previously created point (it DOES NOT eliminate the risk of any spying malware existence in your system, but eliminates the risk that calculating application is trying to steal your seed)
  7. Connect the Internet
If you have an old computer that you no longer use, it would be highly convenient to designate it as the sole offline tool for handling your seed. You will never need to connect it to the Internet again, and even restoring the system to factory settings won't be an inconvenience. If you follow one of the ground rules above to handle your seed, you will stay safe from any Internet risks. The only remaining potential risk will be the method you use to choose your 23 words, along with how you choose to store them (assuming you don't just store them in your mind). Remember that all the UNIX/LINUX based operating systems are by design much more secure than any Windows.