Alternative world types are loosely based on the real world, but aren’t the real world. Something about the alternative world is different from the real world. Steampunk fiction set in Victorian England is alternative. Kate Elliot’s Cold Magic is a good example of how the real world can be different with the inclusion of magic as normal in everyday life and how that has changed history.
The example above is a good one of how you can radically change the real world, by changing history but not changing it so much at it is unrecognisable as based on the real world. Her books are have elements of the history of the Napoleonic Wars, just with magic as part of everyday life and none of the famous people from history like Napoleon being involved. Her work is quite an extreme example of just how alternative you can go.
Most alternative world types simply simply have a few added elements that change the real world; this can be supernatural or it can be scientific.
The addition of a supernatural element that isn’t myth, but is in fact reality is a really great way of creating an alternative world. The Sookie Stackhouse books by Charlaine Harris and the TV series ‘True Blood’ shows a world where vampires, werewolves and fae exist, and the resulting changes and adaptations that humans make as they come to terms with this sudden change of myth to reality. Introducing just one of these elements can create an alternative world. Please, please I am begging, don’t make your vampires glittery.
Changing some scientific elements though, can also create great alternative realities. One of my particular favourites is Stargate SG-1, which was based in the contemporary world but a one where interplanetary and even intergalactic travel was made easy with the discovery of ancient alien technology that spread human culture across the galaxy and opened up earth to the risk of being destroyed by powerful aliens who had once posed as various gods from across human history.
Neither ‘True Blood’ or ‘Stargate SG-1’ were what I would call subtle in the changes. You can make one small change to history and have a massive impact on the world. One of the very best examples of a very subtle change to the real world is ‘Never Let Me Go’ by Kazuo Ishiguro. The development of cloning humans doesn’t have much of an effect on the world, apart from to improve health and to create a steady supply of organs. It does however have a devastating effect on the clones themselves who are as human as us but are only seen as donors and can’t have a normal human life. This one subtle change to the world is sublime.
When creating an alternative world you can do as much or as little as you like to change the real world, the thing I think you need to remember is to do it well.