Step 1: Go to the Options page, linked from the navigation bar at the top of the page. Then go to Personal Information, then Edit your identities.
Step 2: Fill in the information for Identity's Name, Your full name, Your From: address:, and Your signature:. Now click Create on the bottom of the page. To make a change, edit your information and click Change at the bottom.
Step 3: To make this your default identity, go again to Options>Personal Information. Scroll through the dropdown identities and select the one you want, then click on Save Options.