First, get a bunch of experience to expert level in your chosen field of expertise .. eg engineering, maintenance, operations, ATC, HUFAC, medicine, etc., etc.
Second, do a few investigation courses to give you a foot in the door. Google will list the standard institutions, eg Cranfield, ERAU, SCSI, plus a host of generally tertiary institution or regulator short courses ... all expensive.
Third, knock on the door.