I did mine through mostly self- study, and I recommend daily self-study for written topics in addition to what Rick mentions, finishing up in an academy. The trick with the CFI airplane is balancing your factual knowledge (written stuff and knowledge of the maneuvers) with some solid flying skills: eye, hand, & foot-work. It's not easy doing both well, and you can bust the test in either area- truly is a "say what you do, and do what you say" sort of exam. I think that is why the failure rate is so high, anyone can ace a lone written or skill test, but when you have to walk the walk it suddenly gets kind of tough. Don't be afraid of it- you will become a much better pilot whether you pass or fail on the first try, and of course anyone who is serious will pass. Since the flying is nothing new at that point in my opinion the most beneficial aspects of the CFI are
1) the stuff you learn in the books- prepping for the written exam
2) the change in you that comes from learning to instruct other humans bringing the best out in them
I see great value in both these things, especially the latter, as ALL pilots will eventually end up teaching other pilots. Those who had time to get their skills together as instructors have a leg up on the rest who did not give themselves a chance. The outlook as an instructor is valuable even if you do not need logbook padding.