If you are using Bitcoin Core client-
1. Go to console.
2. Type [dumpprivkey][your address] & hit enter.
I didn't use Bitcoin Core wallet yet though but that's the basic.
If you already set a passphrase for Bitcoin core wallet. You have to unlock it first, before you are able to dump private key.
walletpassphrase "passphrase" timeout
With passphrase is your wallet's passphrase (that one you used to encrypt your wallet). Timeout is time, in seconds you leave your wallet opened before it will be automatically locked again.
dumpprivkey yourBTCaddress > Enter
What I do is different. I do not only backup wallet's mnemonic seeds/ private keys, but also I do two things simultaneously.
- Backup mnemonic seeds/ private keys.
- Backup wallet files regularly. I usually do it monthly.
In case you lose your mnemonic seeds/ private keys, you will still be able to access your funds through wallet backup files. The only thing you have to do in that case is remembering your wallet passphrase. Passphrase should be set as strong as possible and
non-reuseable one.