It's the same way you would otherwise. Wherever you volunteer, they'll sign off on the hours you volunteered for. I don't think colleges need much documentation beyond that, and it'll just be a mention on a list on your app.
...I think, as it may vary depending on school/college app. Ask at the school nearest you!
As the parent of a homeschooler, we never got people to "sign off" on any forms. My son just kept track of his community service via his resume. When it came time to apply to college, he wrote about his service in essays and the Common Application, and got at least one recommendation from someone familiar with his service.
I don't think college expect non-traditional students to have the same procedure for documentation as students in traditional public or private schools.
Oh, sorry! I meant more that if you were volunteering for a particular organization, they may document your hours for you, and be able to vouch for you doing the volunteering as a result. Otherwise, you keep track of it yourself and list it on your app/resume. That's usually been the case for my volunteering, but as sbjdorlo said, YMMV. The colleges don't actually expect the documentation, just that you'll list it on your resume/app.
