The unspoken assumption (when it isn't explicit) seems to be that a DM will design at least a continent, decide on geography and terrain types, maybe even political entities, trade routes and epic histories.
Yeah, that's not how I started this campaign. First it's a ton of work. But more importantly, it makes a bunch of decisions before play that shuts off the possibility of shaping the campaign as we go, as I receive input from players, etc. It limits the ability to adapt and react. So here is quick and dirty way to put that off till later.
You have a mysterious pylon appear near civilization. In it is a control room with little dioramas of various places that appear to be ruins. Working various levers/switches allows for travel to these locations. There is either a similar pylon at the location, or a simple mechanism for returning directly (break a crystal, chime a gong). (yep, like video game fast travel-- though you might decide later that some of these locations are actually in different times or planes [they didn't fit in your conception of this campaign world]).
Now you can have 2-10 locations that are whatever seems coolest to you when you start, or, whatever module you want to use, and you don't have to worry about where they're located in relation to each other or how they fit into your world's history. If players decide to come back from a location the old-fashioned way, by land/sea travel, you'll build the world as they go. Otherwise, you have time to decide what you want the greater world to look like while the players are exploring these hot spots. Essentially it's a stalling technique that allows fun play now that can still be situated in your world to whatever degree of simulationism you desire later.