Everyman! Just make a work of fiction you can feel proud of. You will find your characters, places and events start to take on a life of their own. It does not always happen, but if you keep an intent to do so (or no intent) throughout the process, there is a good chance. And you do not have to be particularly creative either, plagiarize history, try your hand at someone else's incomplete fictional universe; it all depends on how much you want to do.
Separate is easy enough, imagining your new creation has no ties to anything but your imagination, while getting past your thoughts on just how much... thoughts can affect the world is tend to be tough. Generally speaking, the closest you can get is an astral tour of your newly made locale; or if it becomes popular, rampant fans mimicking your characters (much more disturbing than when it's someone else's). And anyone can see the effect of Sherlock Holmes or StarWars on the world, such fans as Trekkies and Otakus; there's even books on the subject, like
The 101 Most Influential People Who Never Lived.
Or, if you're more technologically minded, you could make a virtual environment with it's own simplistic physics and digital life-forms. There's plenty of programs, programming books, and simulations already. Sure, that's not exactly an equivalent (yet); but it's here, it's possible, and one day with faster processors, it may be indiscernible from the rest of reality.