I've just claimed it. The form is online. You fill it in online and print it out, then take it round to the Child Benefit claimant (usually Mum) to sign. Very straightforward, and it's backdated to 2011.
I wondered if it took credits away from the parents but essentially a working mother is getting two lots. All you do it get the Child Benefit-mediated bit transferred to you. No-one loses out.
As I ditched two days of a job I hated anyway to look after my DGDs, I lost all employer NI contributions. Now I've them back and they count towards my state pension.