On a first draft, give yourself a daily minimum word count: something you can easily do. The amount will be different for everyone, but I usually aim for 4 sides of A4 – 1,000 words. If I feel like doing more, that's fine, but it doesn't mean I can do less tomorrow; I have to do at least 4 sides a day. While I'm doing this, I don't look back or edit or even wonder if what I'm writing is any good. I just write 4 sides a day, every day, picking up where I left off yesterday, always moving the story forwards.
Every time I stick to this, within a couple of months, an amazing amount of writing has somehow happened. That's how I wrote the first drafts of Varjak Paw and Phoenix, and that's the secret of how to do a first draft. Don't judge it, don't worry about it: just get those words down on the page, whatever they are. Momentum is everything. If you try to edit while you're writing a first draft, chances are you'll never finish it. But keep moving forwards, and you'll reach the end before you know it.