Talk to your local School Board Superintendent early in the process. You will likely need the Superintendent's approval for the age exception, but also remember that Superintendents' decisions trump the official rules. This is the person who can review your homeschool and course completion documentation, approve a preparation course and authorize any necessary testing accommodations.
Apply to a college if your state requires this for testing. Virginia requires that you have completed your coursework, while Arizona only requires an affidavit of intent to homeschool. Verify state requirements provided by the Superintendent, because homeschool law changes frequently.
Complete an approved GED preparation course with sample tests in each of the five areas. Repeat the tests until you are satisfied with your proficiency in each subject and are comfortable with the testing format. You should attempt several of the tests with a timer, so that time limits do not make you feel rushed on test day.
Apply to take the GED test. The American Council on Education's website has an application, test dates and a list of fees. Contact the testing center in advance to discuss any center-specific requirements and to alert the Examiner that you will be testing as a homeschool student.
Assemble any additional documentation that your Superintendent requires. You will need valid photo identification with your birth date for the test examiner, and you will need the signed approval forms on test day.