You'd go to a post office. If a homeless person can't figure that out, then they are uninformed. The US system doesn't work with uninformed voters. So, that's a win IMHO.
"Convinced that the people are the only safe depositories of their own liberty, and that they are not safe unless enlightened to a certain degree, I have looked on our present state of liberty as a short-lived possession unless the mass of the people could be informed to a certain degree." --Thomas Jefferson
It's easier to be informed of candidates' position than the logistics of voting. Free and discarded newspapers are everywhere, and cover the former extensively, but the latter only marginally if at all.
Case in point, at least two HN readers (OP and myself) didn't know homeless people could vote by mail at the US Post Office, but I know at least one of them is very well-informed on the issues.