It's the same advice you'll get almost anywhere on the internet, but the most important part about writing is, well, writing. Don't worry about the quality of first drafts. The first thing you write down is (almost) always going to be shit. Writing is all about editing and revision.
The hardest part of writing is motivating yourself to actually write that garbage first draft. It's hard to get pumped to write something you know well be crap, but without it you have nothing to polish, nothing to revise.