Frequently unclaimed because carers don't realise the hours they already give qualify — but it's worth checking the trade-offs before you claim.
This is the step that trips people up most: Carer's Allowance isn't based on how much care you give in isolation — the person you care for has to already be receiving one of a specific list of disability benefits (PIP daily living component, DLA at the middle or highest care rate, Attendance Allowance, Adult/Child Disability Payment, and a few others).
If they're not yet claiming one of these, that's the actual first step — help them apply for PIP or DLA first. Once that's awarded, Carer's Allowance becomes possible.
Carer's Allowance is taxable, and it can reduce certain means-tested benefits you or the household already receive, because of how "overlapping benefits" rules work. For most carers it's still a net gain — plus it protects your National Insurance record even in months it doesn't pay out — but it's worth running the numbers on a benefits calculator first rather than assuming it's simply extra money on top of everything else.
Online is fastest. You'll need your National Insurance number (and your partner's, if you have one), bank details, employment details, and information about the person you care for — including their date of birth and address. You can backdate a claim by up to 3 months, so a slow start doesn't mean lost entitlement.