When you open the FM18 Pre-Game Editor, you are greeted by a stark, spreadsheet-like interface. Do not let the minimalist design fool you; it is incredibly powerful.
Are you tired of your board rejecting a request to upgrade youth facilities? Just change the club data.
This is a separate application installed with the game (found in the Steam\steamapps\common\Football Manager 2018\editor folder). As the name suggests, it allows you to edit the database before starting a new career.
Unlike "in-game editors" that allow real-time changes during a match, the pre-game Editor works by creating a custom database file. When you start a new career, you load this modified database alongside (or instead of) the default one.