Step2: Open the Polkadot.js extension and click "+", then you can see the following options.
Create new account: You can make a new account from 12 seed phrase. Derive from an account: You can make a new account from your existing account. The same seed phrase is used to make another account. Import account from pre-existing seed: If you are using other wallets such as Polkawallet and Fearless wallet, you can import your account to Polkadot.js by using the pre-existing seed phrase. Restore account from backup JSON file: Another way to restore an account is to use a JSON backup file.
Step3: Click "Create new account" and check the seed phases, then check the blank and click "Next step".
Step4: Decide your account name and password, then click "Add the account with the generated seed".
Here, you have completed the creation of your account and your address can be used for all Polkadot networks such as Astar and Kusama. Native Substrate address starts from 5 and Polkadot address starts from 1.