Sometimes a large project like a novel or screenplay or a non-fiction book can seem so daunting that it's difficult to get started. Here is my suggestion for a "map" of one way to break the process down into manageable steps toward your goal:
1: Random notes. While you work on something else, keep a notebook or index cards handy or type your thoughts into your smartphone. For instance, for a novel these might be ideas for a character, or a setting, or a line of dialogue, or a plot twist. It doesn't matter if they don't fit together at this stage.
2: Grouping the notes: Once every week or two go through the notes for a few minutes and put them into groups--all the notes about characters go together, all the notes about the plot, etc. This will make your subconscious mind aware of where the gaps are, and you may find that the new ideas you have will begin to fill those gaps.
3: A broad outline or mind map. In this step you jot down the basics. In a work of fiction, this could be in the categories "beginning" (first quarter or so), "middle" (about 50%) and "end" (final 25%). This is still loose, you may decide to move things around later. Also you may realize there are still some gaps, and that's OK, too.
4. The next step depends on your preferences. If you prefer to have a full outline before you start writing, then filling in the gaps and refining the order of events is next. Alongside this you may want to do some more thinking about your characters and their relationships. On the other hand, if you prefer to start writing and trust that you'll come up with things as needed or else fix them in a future draft,then you can go right on to step 5.
5. Writing. You don't need to start at the beginning. If you have scenes in mind, go ahead and write them. Yes, you may have to adjust them later but that's fine. This way you get the flow going.
6. Joining up what you've written. If you've jumped around and written scenes from different parts of the story, now you put them together and determine what's still missing.
7. Filling in the gaps. Now you write the missing bits.
8. Once over lightly. Rather than a rewrite, this is just a light once-over to take care of any adjustments that are required as a result of writing the project in bits and pieces out of order. For instance, you may find that you have one scene set in the morning and another scene that is supposed to follow it immediately time-wise in the afternoon, so you decide which one makes more sense and adjust accordingly.
9. Celebrate! You've finished your first draft!
When you keep your eye on one step at a time, the whole process is far less intimidating. Just stick to the map and you'll reach your goal.
(For more tips, see both "Your Writing Coach," published by Nicholas Brealey and "Focus: how to use the power of targeted thinking to get more done" published by Pearson. Both are available from Amazon and other online and offline book sellers.)