Q: I want to write books like you two do. How should I go about it?
Anne: First of all, you can't write like us, nor should you want to. Writing is an individual accomplishment. Find a story you want to tell. Learn the mechanics of writing a novel or novella or whatever format and genre you want to write. Read a lot of those kinds of books so you know what sells and how to go about formulating a good story. There are all kinds of Internet sources and workshops that will help get you started off right. Then write, write, write. Ask for help if you need it. Ask for someone else who writes to read your material and give you feedback.
Dee: Think of a story you want to tell. Make a brief outline—without a lot of details—of what the beginning is, what the ending is and what is going to happen in the middle. Write out characteristics about your main characters: their looks, their siblings, if any, their jobs, what they want to do in life, why they haven't already done it, and something they as characters would never do (somewhere in the story you will make them do it). Read all kinds of advice on how to write, including what Anne and I have said here, and throw it in the trash, except for the parts that feel right to you.
And then, to parrot my dear sister, write. That's the only way you learn what works and doesn't.