The key idea of unschooling is that children should direct their own education, choosing their own teachers. They will find out how they learn best, because they have a natural curiosity and desire to learn. Too often, this passion for learning is stamped out of them by being taught things they don't want to learn, and coming to see learning as a passive, dull process, which is directed by somebody else, not by them. The directing could come from a state school, a private school, a private tutor, or their parents.
The proper role of the parent is to facilitate learning, primarily through imparting on their children how to teach themselves, or how to find teachers and resources. A young child may ask their parents 'how does a car work?' or 'when did dinosaurs walk the earth?' or 'what makes trees grow?'. Can you answer these simple questions in any kind of depth, without googling them, or asking someone, or going to a library to read up on them? If you honestly answer 'I don't know; but I know how to find out,' and then explore the answers to those questions along with your children, they will soon be teaching themselves things without your assistance. When they get a bit older and start to explore higher-level science, they may want to take some formal education, perhaps a university or specialist private school or private tutor, and if they do they should be encouraged.
So to address a couple of common concerns of parents who do not send their children to school, there is no need to hire a private tutor, and doing so may even harm the child's ability and desire to learn. And there is no need for a parent to be all-wise, able to answer any question; they must only know how to find out the answers, and enthusiastically encourage their children to seek out the answers.
David Friedman recognizes that a large part of his education came from places other than school, and he unschooled two of his children: here he talks to Stefan Molyneux about it.