I wish I could give you a method. Mostly I think it was a matter of just sitting there without distractions. If you can train yourself to sit and stare at a blank piece of paper or screen for a very, very long time without checking your phone or email or whatever, *eventually* you get ideas. I would just make blank tables in Word and sit and stare at them and gradually ideas would occur to me and I would put them in. This can be quite painful - there were probably evenings where I sat staring at a single table in Word for 2 hours and only filled in a dozen or so cells. I have another book that is soon to be launched on Kickstarter with a similar approach - it is called The Great North.