Since I switched to the wider sketch card format I have been using my Cintiq 21UX to digitally draw the roughs for each card. I then print out those rough drawings and transfer them to pre-cut bristol using a lightbox.
Drawing the roughs digitally allows me a lot more flexibility and options for composing images than I ever had when I was still sketching them out on paper, and this results (I believe) in a better product for my customers. I can draw each visual element on separate layers and then compose them afterward, allowing me to experiment in ways not possible in the analog world (and much faster, too). If I need to rescale, flip or rotate an element of the drawing, I can do all of that with ease. And because it's being drawn on a Cintiq, it's all drawn by hand.
Here are some samples of the digital roughs (drawn in blue, just as I did with my analog roughs) and the final cards drawn with Micron pens and colored with Prismacolor markers:
As you can see, some of the digital roughs are very rough. I am mostly concerned with finding a good composition and drawing the basic shapes and figures. The drawings are solidified and refined when I transfer them to the actual sketch card and even more so when I ink the pencil lines. But drawing the roughs digitally is the best way of starting the process because it allows me to work faster and explore options in ways that I never could by drawing on paper.
For years I worked in an analog to digital workflow. With the Cintiq in my toolset, I've reversed the process and it's improved the way I work in many, many ways.
-Otis