To write you must have your butt in a chair. I guess you could write laying down, but that always ended up with my pillow taking advantage of me for a couple hours.
Not a very productive end to a writing session.
After successfully seating yourself in your favorite chair, at your desk, in your car, work break room or anywhere else you want to sit you need your supplies.
•computer
•phone
•notebook
•scrap piece of paper or note cards
•stylus
•pen or pencil
•colored pencils
•crayons
Basically anything to write with and to write on. Then write. Anything. It doesn't have to be in order. Can be a group of notes. Can be a complete chapter. Maybe it's a description of a character. Just write...your butt is in the chair