How do I know that I'm covering all subject material and in the right age appropriate sequence?
It is truly impossible to cover EVERYTHING. Even the public schools do not cover everything, however it is important to at least look at the Typical Course of Study for various subject areas and grade levels when selecting and/or developing a homeschool curriculum.
Missing some key elements will not make a difference if your child has developed a hunger for knowledge. They will be able to pick up anything that they missed fairly easily if they have learned the valuable ability to ask questions and do independent research. The most foundational principle of education is not to "pack in" as much information and facts as possible in 12 plus years, but to ignite a desire for learning.
Education is not the filling of a pail,
but the lighting of a fire.
~ W.B. Yeats
Scope and Sequence Guidelines can serve as a very helpful roadmap in teaching subject areas. For example, a child has to learn letter recognition, followed by phonetic sound associations, before they can read or write. Skills and subject components build progressively upon each other and when this sequence is followed in many subjects, the child will learn naturally. By using a "roadmap" the homeschool parent is able to look and plan ahead. They can also focus on components that their child may have a special interest in. If a child is struggling with a specific component, they can easily retrace steps and review a previous component before introducing the next.