If the three-step process is new to you, here’s the most important tip: Avoid editing during the drafting step! You can even visualize yourself escorting your “inner editor” out of the room and locking the door!
Explore the ideas you listed or brainstormed or bubbled. You don’t have to write from beginning to end. You can start with an idea that seems easiest, the one you feel the most confident about. Or you can start with a hard concept, get as far as you can, move on, and come back later.
If you have trouble getting started, try freewriting: Set a timer for 10 or 15 minutes. Start with a simple sentence and keep writing. Don’t stop. If you can’t think of any words, draw circles or write la-la-la until the words start flowing again. When the timer goes off, you can keep going if the ideas are flowing. Or you can take a break, then freewrite for another 10 minutes on another word or idea.
The Writing Process website at MIT has a good step-by-step guide to freewriting.
Once you are done drafting, you can move on to editing.