Before you start you need to have set up an import of the email address in Gmail. Don't know how? Don't worry ...
- Open Gmail
- Go to the cog on the right hand side and click settings
3. At the top click on the tab 'Accounts and Imports'
4. Check that the email appears in the section 'Send mail as'
5. If it does not then click add another email address
6. Compose an email
7. At the top there should be a grey arrow by 'From', click to choose which email address to send from