I can only share with you my experience of writing a science fiction novel and my thoughts how this relates to writing a fantasy novel. I think there are some common elements for both genres, but also some differences. For science fiction (especially hard science fiction) you need to research the science part very thoroughly, which takes a lot of effort. In the case of fantasy there’s one major aspect which seems key to me: you need to be consistent about the objects and rules you create. Very often you don’t have to explain why certain rules exist, but you have to explain how they work. What both related genres have in common is that all characters have to be portrayed in a consistent way in terms of applying the rules and how the world around them behaves.
Keeping a longer novel consistent is very hard, especially when you apply changes or start deleting portions of it (which is very important, because you want to keep the good stuff only). So how do you get past the need to “know” things, let go and stop worrying about whether your story sounds realistic or not? In my opinion you can do this after your general framework has been laid out. While it’s good to keep a journal with all the meta information, it’s also worth memorizing the most important aspects. So whenever potion A is mixed with potion B the magic dragon will… and so forth. If you change any of this (which you can if it makes sense), it’s a good idea to explain why.
When your general idea is somewhere in the back of your mind, just start typing away like crazy. Don’t worry about errors or missing words or whatever. Stop worrying completely! Let it flow and flow and flow. Enjoy the sensation of the moment. You can worry about the details later. There are many good books for aspiring writers with lots of ideas and tips. There’s one book with excellent advice I’d like to recommend:
“Sol Stein on Writing” – A Master Editor of Some of the Most Successful Writers of Our Century Shares His Craft Techniques and Strategies. The paperback version is available for less than $15. A very good investment, I think!
On my website I’ve documented some of my other experiences. If you’re interested let me know and I will send you an PM.