This video will walk you through how to add an alternative email address to your account so you can send from an alternate email. For example, I can send an email from "cfuller@charter.support" even though I'm using my "cfuller@bhope.org" account.
Click on the Settings Gear Icon
Click on "See all settings"
Go to the "Accounts" tab
In the "Send mail as" section, click on the "Add another email address" link
Enter the email address you want to send from.
Go to that email account and get the verification code or click the confirmation link.
Save and close your settings
Compose a new email
In the "From" section, click the dropdown to pick the email address you want to use.
Also you can set up custom email signatures for each email, which is covered in the video as well.