[muffinman26]

By hand on paper.

I don't really see the point in worrying about data leaks. A CPA who leaked your data would have their reputation so completely destroyed I doubt it would be worth it. That's 5 years of expensive and high-energy education to throw down the drain. Plus, the IRS system is probably a poorly secured and very profitable target you can't do anything about.

The real problem is that software has bugs. If I'm going to have to go through the trouble of double-checking everything by hand anyway, it makes sense not to bother paying for software.

The useful piece of advice is how to learn to do this. Volunteering for the VITA program will give you a 2 day training on how to prepare taxes, in return for spending one day a week helping other people file their taxes for a few months. (Which is free spaced repetition and practice, anyway.)