How do I add an alternate airfield?

When you first create a plan, the final waypoint will be assumed to be a landing point and will automatically display the landing label.
Tapping on the last leg of the plan turns that waypoint into an alternate. The alternate label will appear underneath the waypoint name, and the landing label will move to the destination above.

The image below shows an extract of the Plan Sheet with Horsham (YHSM) set as the destination and Nhill (YNHL) as an Alternate.