Most of the learning in school is taken from the material in the text books. So home schooling only requires an adult facilitator that can assist on moving that book knowledge to the brain of the student. Now that in itself is a skill that not every parent has. In those cases, the student, if successful, will figure out how to learn from the books.

Also much of home schooling is done as a correspondence course, which typically comes from a private school (these schools, usually religious in nature, have regular classrooms, and have home-schooled students which are considered part of the class, and they have periodic meetups available for field trips and stuff). In these cases there is a teaching staff to back up the parent, which provides homework and testing material, and sometimes phone support too.