With my son's recent decision to attend the University of Mary Washington, now seems like a good time to brain dump on what we learned getting our first homeschooled kid into college. This may not be well organized...
Start keeping records when they start "9th grade." We had to take an archaeological approach and dig up what our son had worked on for the previous 3 years, because we weren't tracking it closely. It turned out that by the time he started his "senior" year, he had already completed all the requirements for a VA High School Diploma. Of course, he'll never actually have that piece of paper.
Think outside the box. All those hours your kid spends writing Minecraft add-ons? That is a course in computer programming. All those hours playing guitar are music class.
Take advantage of volunteer opportunities. Your homeschooled teen has free time during the day with no competition from the hordes that are in school. Use it! Find opportunities to volunteer doing stuff related to career interests, or just doing good works. My son the soon-to-be History major did volunteer primary source research for a NPS Historian that resulted in co-author credit on a published article when he was 16. Do you think that looked good on college applications?
Take the SAT in the junior year so that you have time to address any deficiencies. However, don't be surprised when the SAT score exceeds your wildest expectation and your homeschooled teen decides that he won't be taking the test again. You can get into school without taking the SAT or ACT. We didn't go that route so I don't have much to offer if you take that road.
A high SAT score plus a homeschooled background = lots of interest from colleges, especially small liberal arts schools. High verbal scores will lead to many colleges waiving the application fee and essay requirements. Take advantage and do the 5 minute application anywhere that offers it. We made a mistake here. We got picky, and quite frankly, a little lazy. High verbal scores may also get you free credit hours by exempting the student from Freshman English. (I assume a really high math score comes with similar benefits.) At a public school, getting to skip a 4 credit hour English class is worth about $1200. Seemingly very expensive private schools have lots of money available to hand out, so don't dismiss them when they send the glossy brochures to the house.
Some colleges will insist on grades. Don't fight it. Just assign grades. Your assessment is no more arbitrary than what a teacher puts on a school report card. If your kid is 95th percentile on the SAT math section you can quite legitimately assign an A for every math course you are claiming. No school will question it. In our local public schools "SAT Prep" is an actual class. If the school kids can claim it, you can too.
Make a transcript that looks familiar to college admissions folks. Make sure the admissions committee can look at the transcript and relate it the high school graduation requirements for your state. Supplement it with a written description of what each course covered. For more traditional subjects where you may have used a textbook or curriculum, you can probably cut and paste what you need from the publisher's website. Off the beaten path or unschooled stuff is limited only by your ability to put into writing what you did. Writing these descriptions is a job for the homeschooled student, not the parent! My son's supplement was 14 pages. Also, apply early action or early decision to your top choice schools. Those kids often get priority for scholarships.
Check out the online or book form AP Test study guides for any subject that you think is possible. Again, it can be free college credits. You have to register for the AP by early March of senior year, usually through the public schools. So check the web for dates and don't miss it. AP scores are not used for admissions - only for credit. So there is no reason to take the AP tests early, unless you are taking more than 2, as they only allow two at time.
Think about utilizing the local community college during the junior and senior years. It can be a great way to take foreign language or lab science courses that aren't as easy to handle on your own. In some states, high school age kids can attend for free. Even if you have to pay, you will be banking college credits at 50-75% off the price of the same credits at a public 4-year school. Between community college, AP tests, and SAT exemptions, your kid can start college as a sophomore. At $25k a year for a public school in-state, that is some serious savings.
Above all else, don't stress. Don't believe the horror stories about colleges being hard to work with when you are a homeschooler. Among small private schools, I found the opposite. They went out of their way to be sure my son qualified for available scholarships. I think just about every Division I school has a process in place for homeschoolers. So check the web sites, they will tell you exactly what you need to provide. The entire process was actually much easier than I expected. Colleges really do like homeschooled kids.