I'm typing it out now, tapping at the keys as I work out how things will go. Usually I have the barest outline - just a few notes of things I know will occur, whether it be positions, or phrases, or a girl in particular.
I know I'm reaching the end, and so I've been looking back at Invention II. It's sat there for years and I know where it's going to go. I think the hardest part has just been getting the time between work and family and so on and so forth to get it done.
The life of an erotic author can be a lonely one. It's not like you can tell people "Oh what do I like to do in my spare time? Well sometimes I write up stories about girls being seduced, or raped, or impregnated, or some combination of the three." Not exactly stuff you can bring out at a party as you discuss philosophy.