Keep in mind, all writers are different. Each one has their quirks and habits, even when it comes to writing a first draft.
Let's take the first draft of a novel.
After I get an idea, and after I've decided that this is the next novel I'm going to write, how I continue depends largely on whether or not I have the first few sentences or not. If I do, what flows next usually just comes to me. If not, then I imagine where the story starts and just start writing.
Bottom line: I put words onto the page. Later on, once the first draft is complete, I may go back and change it, but when I'm writing the first draft, I don't go back and change anything, even if I want to. I resist all temptation--I may write down what it is I want to change, just in case it does make sense--and plow on until the end.
Most writers typically write a certain number of words a day. I probably could nowadays, since I use Microsoft Word, but when I first started getting serious about writing back in college, I wrote my first two novels on a Brother word processor. The best I could do then is write a certain number of pages. To me, a page goal is more attainable to me instead of a word goal.
But, I stress, this is the way I write. You write however you see fit.
I also don't outline ahead of time. To me, outlining is a waste of time--sorry, if you're a huge proponent of outlining. I have to admit, the first novel I ever completed was largely outlined, but it was very limited in scope. I used a balloon method of brainstorming, which looked a lot like this:
I usually used this method to keep everything straight. Then again, I learned much on my first novel--what to do and what not to do--that I've never really used this method since.
How do you write your first drafts?