Someone told me I can use SHA256 to hash and get the output which is 64 characters(256 bits) and that is the private key. But I tried it on one of my unused wallets and used its mnemonic, and then used the SHA256 method but the hash generated is not the same with the original private key of the wallet.
What he (
that someone) described is a "
brainwallet" - it's basically using the output of SHA-256 hashing algorithm that has the same length as a private key.
It's a 'dirty' way of creating private keys and shouldn't be used at all.