1. Clone this Gist.
2. For card sizes other than A5, edit the size value in @page, and the height and width properties of body.
3. Add contents to each face. The simplest approach is to add an image called front.png of the same dimensions as the card.
4. Generate a PDF from the HTML + CSS. If using Prince, it's as simple as prince index.html card.pdf.
5. Take the PDF to a printer, and ask them to print as many copies as you need.