Working out what comes next

I'm currently just getting started on the third, as-yet-untitled book of the Frontier Magic trilogy, and the first step of that is working out the plot in more detail than "they explore the Far West to find out what happened to Lewis and Clark and what's up in the Rocky Mountains; various characters solve assorted personal and romantic problems" which is pretty much what the original outline said. So I thought I'd talk about how I do that. Warning: I'm trying hard not to, but there may still ...