Sign up online for the SAT at www.collegeboard.com. Here is the exact address:
They take the test at their local high school. They can choose any school on the list - some prefer to take the test at a private school. I don't know if it's possible to refuse a homeschooler, but I've never heard of it.Here is their page called "Homeschooled students and the SAT." The parent can read more about the test and how it is administered.
This page proves how homeschool friendly the test is - they are VERY used to seeing us, it's not unusual to them at all.
You may be wondering how anyone can take mommy grades seriously. I know you are worried, but your home-made grades really do matter when homeschooling high school!