When you create a character, you give them life; they become like your children. You create every hair on their head, give them a name, and they become real - they are born.
When I was expecting my kids, my husband and I put a great deal of thought and research to find potential names for our children. My characters practically name themselves. I just close my eyes and there they are. As much as I plan every other aspect of my life, this may seem uncharacteristic, maybe even slightly unorthodox. Does it work? I'll have to let you know.
Once you have characters, then you need to tell their story. How do you know what they want to say? It's not like they can tell you... or can they?
How to approach the planning of a book itself? Is it the same as planning a project? With project management, you consider scope, schedule, and cost. When writing a book, you consider character, timeline, and book length. You can take many different approaches, and there is even software to assist in the creative process. What works? Is there a magic formula? I wish there was... if so I wouldn't have half a dozen unfinished stories bouncing around in my head.
Can you design a process to plan your book? Perhaps such a process already exists? (I picture some sort of flow chart). Will a well-planned book be even better than a book written on the back of receipts you found in your purse (not me) or on a paper towel (guilty as charged)? There are no easy answers. Just do it your way. But, most importantly, just do it! The only thing standing in your way is procrastination... and on that note...