Make Writing a Multi-Pass Process
Create an outline of what you are trying to write. Even if you do not follow it, the act of creating one lets you get the big picture of what you are trying to write out on the page.
Or if you just want to create as much as possible as fast as possible then just do a brain dump. Horton calls it a splat draft. You don't think of any form. It is just a stream of consciousness draft.
Coming back and filling the pieces or rewriting what is already there is the next step with either of these options. Hence, multi-pass.