Take a look at the Childcare Choices website for information, and ask your particular nursery how they do things, as there are wide variations in how thee scheme is applied in practice.
First you need a nursery that participates (not all do) and see how they offer the funded hours. You can claim the hours from the start of the term following the child's third birthday, not the moment they reach 3yo. The 15 hours are pretty much automatic in most cases; you may get 30 hours according to certain qualifying criteria. In some cases, if you wait until they are eligible for the funding, you may not get a place at all.
The 15/30 hours are based on term-time only attendance. You may be offered a "stretched offer" of fewer hours per week, 'stretched' over the whole year. You will not necessarily be offered the days/times you want.Childcare providers can offer their own choice of 'funded sessions'. You may be able to take up further hours that you pay for.
Other 'additional services' may be available, for which you pay: meals, snacks, outings, music classes, and so on.
Please remember it is not "free". The childcare provider is almost certainly subsiding the place, as the funding levels are dreadfully low and in many cases do not cover the cost of provision.