Round 1. Start with an idea of a background or two. It's got to be either something I already know about, or can access research material easily, or fancy doing some research on.
Round 2. Think of a few good plot points to use as markers. Woman starts affair, woman ends affair, woman blackmailed by lover.
Round 3. Start thinking about the answers to the questions the plot points pose - why does she have an affair? who with? does her lover have a partner? why do they want an affair?
Round 4. Start writing.
At some point I'm going to get stuck - it might be 10,000 words in, it might be 50,000 words in. But I know roughly where I'm heading for, and what plot threads I'm carrying.
Round 5. Get out some index cards. Write one for the last scene I've written. Write one for the scene I'm aiming for (which is the next big plot point eg they break up or he comes back).
Round 6. Lay the two cards out on the bed with a big space in-between and stare at them very very hard. What I'm doing, apart from scaring the cat, is trying to work out a logical way to get from one point to another.
Round 7. As I get ideas for linking scenes, I write them out on index cards and cautiously place them between the start and end point. Using index cards means I can shuffle the ideas around more easily or add bits. I might get ideas for extra scenes that come earlier or later, and they can be noted on cards as well.
Round 8. When I'm happy with my linking scenes, I write them up as a list and attach it to the end of my novel for reference.
Round 9. With the index cards by my side I get writing.
Write until stuck, at which point repeat Rounds 5-9.
I'll repeat this process until I get to the end of the novel. As I write more - I'm writing Novel No 6 at the moment - my forward planning gets scrappier and scrappier and the bedroom and office are littered with scrawled index cards. I do wish I was a planner - it would feel as if there was a safety net underneath me - but the two occasions I plotted out the novel from start to finish before I began writing, I completely lost interest and never finished them.