I drew out the elements in pens (Staedtlers and Stabilos), scanned them in Photoshop and then image traced them in Illustrator. I was pretty impressed at how the image tracing was able to preserve that "felt pen" look. I changed up the colors in Photoshop and added filled in areas (everything was pretty much only outline in the original drawings). Here is a close up. You can see how Illustrator was able to preserve the look of the lines overlapping.