Recently one of the moms at one of our homeschool groups held a talk / discussion about applying to colleges after homeschooling K-12. My own kids aren't there yet (being only 11 and 7) but I thought there were plenty of useful things to pass along.
Basically, you make up your own transcript. This will often be a combination of classes taken in some "official" format (online high school classes or community college), credit hours given based on the student's own home study, extracurricular activities (which for homeschoolers can be just about anything), and any external tests / proficiencies (such as AP exams or language proficiency tests). There are various transcript formats -- these things aren't standardized among schools, either -- and you should experiment with which layout looks best for your child.
This mom's child had applied to 5 colleges using a homemade transcript, and none had asked for any sort of verification of anything in the application. (3 of the 5 sent acceptance letters and the other 2 are still pending.) If your kid reads 25 books on a subject, and they've been discussing them with you, you can call that a class, and no one is likely to question you.
One of the moms in the discussion found this a bit odd, especially since a homeschooled kid -- who can keep reading and studying until (s)he has 99% comprehension of the materials used -- is very likely to get an "A" in any home study course you list on the transcript. She said it seemed odd to just say "Mom gave me an A".
The mom leading the discussion said that of course there are subjects which don't work well, for any given child. A kid might love physics and despise chemistry, but as a homeschooler one is free to list an "A" in physics and simply leave chemistry off the transcript. You could potentially list environmental studies, astronomy, or paleontology as a science class instead.
At some point I chimed in that school grades don't necessarily have any more integrity than an A your mother gave you, because a schoolkid can cram for tests, pass them with flying colors, and remember absolutely nothing 2 weeks later. This was what I often did in school. There are rarely cumulative final exams in high school, and for obvious reasons-- kids would flunk them in droves. A homeschooled kid who has pursued their own interest in some area is likely to have excellent retention of information relative to a kid just trying for the "A" in their high school class. And it goes without saying that you'll remember (say) a Stephen J. Gould book far better than a textbook. To paraphrase George Bernard Shaw, those who can write well don't get stuck writing textbooks.
Colleges do often want some sort of third-party verification of abilities from homeschooled applicants. The usual PSAT, SAT, ACT tests are one source; there are topical SAT tests one can optionally take (say in world history or higher mathematics); there are tests offered by organizations such as Sylvan Learning Centers which can be useful; community college or online high school classes are a source; and letters of recommendation from academic sorts (tutors, music teachers, etc) are yet another. Some colleges have homeschoolers fill out an extra, supplemental section of the application.
Rather than finding the discussion stressful, I actually found it very reassuring, because this particular family are not just homeschoolers but unschoolers. I found out that on one of the standardized tests, students are asked to copy out a sentence, in cursive, which affirms that they are who they say they are and have not cheated, or whatever. This was the most time-consuming part of the test for this kid, because they'd never studied cursive and were writing it for the first time. I, too, have been thinking of giving cursive a miss (both my kids type faster than they write and always have)... but now I know that my kids should at least be able to write this particular sentence in cursive!
Lastly, it occurs to me that if your kid was 17 and you didn't think (s)he was quite ready to apply to colleges, he/she could always do a little test prep and take a couple of community college classes and pick up an additional extracurricular activity, and then apply a year later. For homeschoolers I think it's often a more gradual transition from high school to university. For example, one of the high school-aged kids in our group has a volunteer research internship at the local university, even though college is still a couple of years off. And many of our kids take community college classes while still in high school.
The mom who led this discussion had this final advice: Don't think that homeschooling in itself will make your kid stand out, but don't worry that it will be a hindrance in any way. I'd add that in the recent past, statistics showed that colleges and universities were accepting homeschooled kids at a higher rate than schooled kids. I imagine that's partly because homeschoolers have the time and the freedom to study unusual things and/or to pursue unusual hobbies, which does make them stand out. And there's evidence that homeschoolers participate more in the community and tend to be more politically active than schooled kids, which may matter to some colleges.