This is some of the best advice on process that I've come across. So simple, but the copy and write-over step makes it feel like you're always standing on something firm, instead of flopping around on ice the way writing usually feels.
It comes from the indispensable Warren Ellis newsletter Orbital Operations:
A thing I learned from John Rogers is to write-over. Each stage of the job - beat outline, treatment, screenplay -- is built on the other. Copy the rough beat outline into a new document and rewrite and add and expand until it's a treatment. Copy the treatment into Final Draft and rewrite and add and expand until it's a screenplay. That way, you only start with a blank piece of paper right at the very start of the process. Every other stage comes with scaffolding and a base coat. Works for prose and comics too. Try it. Just copy the previous document into the next one and start rewriting and adapting it. Might not work for everything, but it might give you a leg up.