You can always start in this world and invent a new way of getting into a fantasy world... There are lots of ideas. It could involve water, or could happen while your character (or characters) is sleeping: It could be a doorway that you have to walk on certain stepping stones to unlock...
Then think about the land you want to create. What will its inhabitants be? Will it be similar to ours? Or will it be totally different?
Then, ask yourself why are you going to transport the characters there? Will there be a task for them to perform? A quest? A rescue? Or will they need something from the fantasy world to bring back to our world, in order to save someone or save the world or something?
Think out the general outlines, and just...start writing!
See what happens.