I've not a lot of knowledge on the specifics, but i do know the Gov website is very good in setting things up, apparently it will only take 5 min if you have:
- a Government Gateway account - if you don't already have one, you can set one up by using this service
- your National Insurance number
- access to a mobile phone or landline
- details of one or more of the following - your UK Passport, tax credits, P60 or a recent payslip
It looks quite straight forward, i was unsure if you were on this website -
https://www.childcarechoices.gov.uk/ - which looks a bit more complicate to navigate.