To send a Fax out of Outlook open a new message. In the To address field you will enter NAME@FAXNUMBER.fastfax.local (i.e. email@example.com). The name can be anything and you do not need to add a 1 to the Fax Number.
Fill in the subject line and enter something into the Message body; this will act as your cover sheet. If you have images in your signature you will want to remove them as they will cause the fax to fail.
Attach a single, simple* PDF and click Send.
*The PDF cannot have Macros or other advanced features in it, otherwise the fax will fail.