Well, I wouldn't try to reinvent the wheel. You create and modify as you need to, but if you can find something, or at least part of something that seems to fit pretty well, then that saves you a lot of work. Kind of like not having to create all your own recipes from the ground up.
The answer to your question would depend a lot on what age you're talking about, the child's needs and so on; you may also be required to fulfill certain subject requirements etc. depending on your state.
Ruth Beechick says that you teach the child, not the book; but that doesn't mean that you can never follow anything pre-planned, either. If you're interested in designing your own plan, you might want to look at some published examples what other homeschoolers have done; Rebecca Rupp's K-12 book is a good example. Others would include Laura Berquist's books (Roman Catholic) and the online Ambleside Online curriculum (
http://www.amblesideonline.com.) Any of those might give you something to start with. There's a book called How to Write a Low-Cost/No-Cost Curriculum for Your Home-School Child by Borg Hendrickson; I haven't used it myself but it's been around for awhile. Another good book to read is Mary Pride's Schoolproof--to keep the educational clutter level down.