I usually think of a quick draft and then it under goes hundreds of edits before I'm decently happy with it. My preferred way to plan is to make a timeline and an outline. The outline is the basic story and its history. It will end up changing a million times, but that's part of writing -- killing your lovelies. When the outline is decent enough, I start on the timeline. A historic document for me, that counts down when something happens, why it happened etc etc. This way, I have the whole thing planned. There's also room in between to edit or add things. When those are done, I start writing in the characters, though that should have started before everything else, I tend to write more about them before and after. Sketches, interviews, giving them problems to overcome (even if its not in your story or game, it will help you understand your character) and all that.
But to get pass writers block, I would suggest starting with the characters you made already. Make an interview with each character, ask them questions. Like the point of the story, their drive etc etc. Understanding why your characters are fighting or there, will help considerable with writers block. There's also the most common and helpful; stepping away from the game/story for a while, reading other people's work, looking at landscapes, listening to what is peaceful and inspiring to you and even brainstorming different ideas. This will refresh your mind and help with the block.