Since you are asking how to deal with single private keys properly, the wisest decision would be to create a regular wallet like Electrum and then sweep your private key using the "Wallet menu > Private keys > Sweep" option. Sweeping your private key means you will send all the coins that this private key unlocks to a new address that belongs to the new HD wallet you created which is with a regular seed phrase and which is a lot easier to backup and secure. Here is a thorough guide on how to do that: https://bitcoinelectrum.com/sweeping-your-private-keys-into-electrum/ Also, here is the very important guide on How to Safely Download and Verify Electrum.