Open the Fighter Squadron screen (F7), select the squadron, you want the fighters to be assigned to, doble-click the fighter to assign it to the squadron.
Make sure, the "show all Race Fighters" (upper right) is ticked, or un-assigned fighters will not show up in the list
Make sure, the carrier you want the squadron to be based on is selected (upper left "Mothership"), if you want to land/launch the squadron from this screen