if you are asking how technically to do it, i can not help you. maybe ask it here:
https://bitcointalk.org/index.php?board=6.0but as an end user you can use the wallet that created that Masker Private Key or Seed and most wallets have the option to see the private key of a single address. for example Electrum that i am using and is a HD wallet allows you to do this.
just right click and select private key.
what wallet are you using?
bitcoin-core uses bip32 (
https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki) since i have never used core before i can not help you more, sorry.