You can't. Only safe way is to make up your own key from 21 words and use it as "paper wallet".
Discussion
Wait, how do you mean? Is that possible?
Sure, you need some maths, or better python programming skills.
It's pretty simple, also without the BIP39 Seed phrase... Tip: The bitcoin private key is just a number. You can pick your private keys randomly using just a coin, pencil, and paper: toss a coin 256 times and you have the binary digits of a random private key you can use in a bitcoin wallet. The public key can then be generated from the private key.