The glib answer to that question is, "With difficulty!" But that's neither helpful nor totally true.
But it's a question that's quite relevant at the moment because I'm Between Novels. Moving a Mountain is as good as I can get it for the moment. I've reached the point of diminishing returns; a couple of months spent giving it another revision pass isn't going to dramatically improve it, so it's best to just keep sending it out to agents and publisher. But Revealing Secrets hasn't taken off yet either.
I hate first drafts. For all sorts of reasons:
1. Performance anxiety. The change from working with nicely polished final draft to raw first draft makes me feel I've lost any little writing talent that I might have had. Aarrgghh! This is dog poo I'm turning out! Or that's what it feels like.
2. I have a whole bunch of characters and settings to get to grips with. The first novels I wrote (unsold and unsellable!) were based on characters and settings that had been in my head for many years, daydream friends and locations. But the novels I'm writing now are relatively new, stuff that has just been around for a short time. I have to feel my way in.
3. A new novel has no momentum of its own, I'm pushing it all the way. It's like bump-starting a car. Once the situation is set up and the characters come alive, I can sit back a bit and watch them get on with their adventure. It feels like I'm just recording something that I'm observing. But getting it all going? Not so easy.
4. In recent years, I've been making myself to do stuff that's a little different. That's why I abandoned The One About the War to write Moving a Mountain and that's also why I'm writing the magic FE college book next and not writing book 2 of the hostel series. That would be more comfortable, slipping back into the same world with the same people, but would be very much the same old same old and won't help me improve as a writer. But the downside to getting better is that you realise there's more to it than just writing down what happens in tidy grammatical prose. I now know about voice and viewpoint and, as I don't want to waste hours writing stuff from the wrong POV or with the wrong tone of voice, I need to feel my way around the characters and situation and decide what is going to work.
For instance while I was writing Moving a Mountain and Revealing Secrets was just a heap of notes, I thought it was just going to be a light comedy. But I now feel that it wants to be darker and have truly lyrical passages and scary moments. If it's not going to change mood half way through, I need to signal this from the start. So I know I need to do more subtle things than I've done before.
5. It's difficult to decide when legitimate pre-writing (all the researching, brainstorming, planning, character and world building that a writer needs) has turned into pointless cat vacuuming. When should I just sit down and apply seat to chair and fingers to keyboard and just write? Actually, I think that point has just been reached and I should start serious word production some time this week.
Having said all that, people vary a lot as to writing methods. I hate first drafts and find it hard to get started, but others love the freedom to put down all the new sparkly ideas and really enjoy getting to know their characters and find out what they're going to do. These writers often hate revision, whereas I like the polishing and tidying up because for me the hard work is over, but for others it's only just beginning.