Cover old garage windows
You can build shutters and place over them.
Cover the glass with a static cling window film. Rabbitgoo is my favorite brand.
are you wanting to cover the windows for privacy or do projects with them?
I bought cheap vinyl blinds at Walmart to hang on my garage windows and I can take them down and hose them off when they get grimy
They have a variety of privacy film , like contact paper for your window , at home depot .
Get second hand shutters for a vintage feel or new shutters for a more modern look. They could be placed open or closed.
The answer really depends on why you want to cover them. I would go to Home Dept and buy a 4x8 sheet of vinyl lattice (which comes with two size holes) and plastic trim to build a frame with and make a complete cover to screw over your window. This will destort the view into the window but at the same time allow the light to come in.