1. Open the Polkadot{.js} extension and click on the "plus (+)" button. Then select "Restore account from backup JSON file".

2. On the next screen, select your JSON file from your computer or drag and drop it in the Backup file field.

3. Now enter the password you set for your account when you created it and click Restore.

Your account has now been successfully restored and you will see it in the list of accounts in the Polkdot{.js} extension.