When starting a mission, in the lower left corner is an Icon with a pilot/aircraft, which is used to edit the squadron/flights. After clicking that, there will be an icon of an aircraft, SQUADRON EDITOR. Click that and you can select from the default squadrons.
If you want to make your own, I think you can add a folder under the Fa-18\Resource\Squadron with the name of the Squadron and appropriate insiginia/text file as in the defaults. You should then be able to select this from the Squadron Editor Page.