A dark background, and light colored, reversed letters are all that is needed to “project” your message from dash to windshield. You can do it with a printer, but it’s ink-intensive, so I put my diecutter to work instead. This is white vinyl on black cardstock, but white cardstock letters would work fine, too.
A couple of caveats. It only works when light is shining through the windshield (so during the day and under street lights) and polarized lenses dull the effect significantly for the viewer. But hey, it’s cheap and this low tech heads up display is a cool way to surprise a special someone with an encouraging message (hijack their fancy sunglasses first) or remind yourself of something important.