Does anyone know how to do this?

Select > Flight > Save   on the main top menu.

 

In the dialogue screen that appears, choose a name, then click the "Default" icon button and hit save.

 

However there is a problem is P3D2 at the moment, and your new flight will require you to open it in Notepad and change the value in the [Window.1] section, that says "Locked" from True to False.