Open the Polkadot{.js} extension and click on the "plus (+)" sign on the top right. Then select "Import account from pre-existing seed".
On the next screen, enter your mnemonic seed words.
Once entered, more fields will appear. Enter a name for your account and set a password, then click on the orange button at the bottom. Add the Account with the supplied seed
Your account has now successfully been restored and will appear in the list of accounts in the Polkadot{.js} extension.