To Send As you'll follow the following steps:

Open settings by clicking on the gear in the upper right-hand corner of your screen.

Under Mail then Compose and Reply, scroll down and make sure the "Always Show From" check box is checked. 



Then to send as you'll have to do that when in a new email. Click the drop down arrow beside your email and select the appropriate email address. If one is not listed, you'll need to select "Other email address."