I personally think the CFI schools are a waste of money. You can absolutely do most of the preparation on your own. I recently instructed a CFI applicant who did his initial with the Feds and failed after attending a large CFI school (American Flyers). He passed after my signoff for the CFI-I and the CFI-A.
I would start by creating your own lesson plans using the appropriate PTS as an outline. You can buy lesson plans but I believe you learn a lot more when you create them yourself and use your own words.
Study the advisory circulars and know the endorsements required for various ratings, solo, etc.
Also, study the FOI's. You need to be able to teach "how to teach". You should know the psychology of learning inside and out.
As far as flying goes, you should be able to pass a commercial checkride from the right seat. It's essentially the same ride as a CSEL (if you are doing your CFI-A as your initial). Go practice all your maneuvers until you are flying to standard per the PTS. The hardest part for new CFI's is the teaching aspect. You need to be able to fly and talk at the same time.
Remember, you are being graded on your ability to teach. There is a lot of information and it's impossible to know everything. However, it's an open book test so know where to find the information.
Once you've put in some serious study hours, created lesson plans, etc., practice teaching! Teach your significant other, your kids, your cat, it doesn't matter- practice talking.
This is essentially what the CFI schools do. You can't possibly learn everything there is to know in a couple weeks. I remember in school all we did is practice getting up in front of each other and teaching lessons. You had to essentially come to school knowing everything and they charged you to have an audience. They did cram some information in my head but if I hadn't studied beforehand I would've been lost.
Once you've thoroughly prepared yourself, find a good 2yr CFI who can sign you off and help you brush up for the practical.