Once you have found a school that looks suitable, you will have to register your child; the website will tell you how. Usually it means filling in a form with details of your child (name, age, current school) and yourself (name, address, employment). You may have to pay a non-returnable registration fee.
Some schools accept all applications just a year before entry; for others you are recommended to register your child several years beforehand (rarely any longer do you need to do this at birth). Many schools (particularly boarding schools) are technically fully booked ten years ahead but always have places available for next year because parents make multiple applications. Entry to day schools can be very competitive, especially in London, but the competition for any one school will change annually. Make sure your information is up-to-date.
It is obviously difficult to match a small child to the appropriate secondary school − and the child will still have to pass through the school’s entry procedure; early registration is no guarantee of acceptance. So have back-up choices up your sleeve and make absolutely sure that it is the right school before automatically embarking on the selection procedure.