Go to Polkadot.js extension official download page and select the Browser you are using to install Polkadot.js (Chrome/Firefox)
Depending on your selection, you will be directed to either Chrome Web store or Firefox Add-on store. Download and install the extension.
Once the extension is downloaded, you can choose whether to create a new account or to import existing account:
For creating new accounts:
Click Create new account under the + on the top right corner of the extension
Write down the Mnemonic Seed, check the box, then click Next Step
Give this account a name and create a password for it. Click Add the account with the generated seed
Done!
For importing existing accounts:
Click Import account from pre-existing seed under the + on the top right corner of the extension
Enter the Mnemonic Seed of your existing account and Click Next
Give this account a name and create a password for it. Click Add the account with the supplied seed
Done!