U.S. and California
I have a degree in mathematics. Tax returns and their computations are merely a simple mathematical puzzle, which I easily solve.
I created two spreadsheets, one for federal income taxes and one for state income taxes. The latter is linked to the former because much of the California computations require inputs from the federal forms. Each year, I copy the prior year's spreadsheets into a new folder. I download the fill-in PDF forms for both governments and update the spreadsheets accordingly. I mark in yellow the spreadsheet cells that require new inputs; as I input those data, I remove the yellow.
California provides a Web site where I input my taxable income and filing status. The Web site tells me how much tax to pay. I wish the IRS would do the same. However, it is much easier to input into the IRS PDF files than into the California PDF files.
Since I have a large investment in a mutual fund, I can also get Turbotax for free. I download it and use it to check my spreadsheet results. I don't really like Turbotax because it requires too much irrelevant input and because it does not provide adequate capability to include explanatory attachments.
I print the PDFs and mail them via U.S. Postal Service. I never request certified or registered mail. I mailed my first tax returns when I was 16 years old. I am now 72. I have never had a mailed return go astray.