It’s kinda tricky. Ed at Gin and Tacos makes a very good point about voting by mail (boldface mine):

Many years ago I taught a very large lecture course and to facilitate grading 400 exams I asked the students to write the number of the essay they chose to do on the front of their blue books. I wrote several times on the exam in bold type, Write the number of the essay you choose on the cover of your blue book. After a few experiences I also formed separate piles in the classroom when the students handed in their exams with a sign indicating the number of the essay. This pile is Essay 1. This pile is Essay 2. Etc.

If you’ve taught – from preschool to grad school – you know that no matter how many times or how clearly you give instructions, some students won’t follow them. You can get to the point that most do if you’re persistent, but for whatever reason some of them either won’t read, won’t listen, or won’t process them. Of course in the example above it’s not a big deal. It wasn’t like they failed the exam if they put theirs in the wrong pile, or didn’t mark the cover of their book. In that case the only consequence was me and the teaching assistants wondering why they can’t follow simple directions to make our job a tiny bit easier.

One of the issues – not problems, but an issue to be aware of – with mail-in ballots is that the instructions create opportunities to reject ballots. To use the example of the recent Kentucky primary, ballots were rejected for failure to sign, failure to sign in the correct place, failure to enclose the ballot in an inner envelope before putting it in the outer envelope…on and on. Petty stuff, but stuff that is going to get your ballot tossed if you don’t read and follow directions correctly….

In an election in which a lot of ballots are going to be cast by mail and it is patently obvious that Trump will use every possible mechanism to try to question the legitimacy of the ballots cast, I worry about the potential for these minor, insignificant instructions will toss otherwise valid votes. No voting system is perfect and the rejection rate on mail-in ballots has been low everywhere it has been tried. “Low” and “zero” aren’t identical though.

Here is a very good guide on how to go about voting. And if you do vote absentee ballot, please read all of the instructions.