How to learn about instructing
One good reading book for all future flight instructors is "The Flight Instructor's Manual by William K. Kershener", althought he is an american pilot and some of the maneuvers and procedures might be different in the UK, he has some really good advise. Also, reading on the Flight Instructors manual from the FAA might not be a bad idea, at least you will learn about some of the psicology involved in teaching and how to prepare some lesson plans. The FAA updated the book about 2 years agoo to a big improvement.
An the best recomendation, just take any simple task that you do everyday (let's say walking) and try to teach yourself or somebody else how to do it. Think from the simple to the complex, explain estep by setp procedures, and then try to do it at the same time that you are actually doing that task. It might seem silly but it works!
There are many books out there that talk about teaching, try to get your own set of books (aviation ones) and the more knowledge you have, and more different ways to explain - teach subjects, the easiest it will be to do it your self. remember not two students are the same, so you will have to change your teaching style many times.
Good luck with your Flight instructor training, and just go for it.