1. Go to the "Settings" page and then click on the "Accounts" tab.

Go to the 'settings' link up at the top right

Click on the 'account' tab in the nav bar

2. Then click "Add another e-mail address" link.

3. You will get a popup that lets you select the name and e-mail account that you want to add.

Type in your name & e-mail account

4. Once you do, Gmail sends a verification code to this e-mail.

5. Once you verify the e-mail, go back to your Settings » Accounts page. Underneath the "Add another e-mail address" link, there are two radio buttons that let you select whether to always reply from the same address (the second option) or to reply from the same adress the message was sent to (the first option). Select the first option.

Select 'reply from same address the message was sent to'

And you're all set to go!