Get a Polkadot wallet. Official Polkadot{.js} guide is here.

Website: https://polkadot.js.org/extension

Open Polkadot.js Extension ⇒ Import Account

Open your web browser and click on the Polkadot.js extension icon, usually located in the top-right corner of the browser window.

Select which method to use to import the existing account from:

⇒ Pre-existing seed

⇒ Backup JSON file created using Polkadot{.js} Wallet.