Just photoshop! I was drawing over a sketchup model rough we'd assembled of the geometry of the original level. For this particular piece I just used a typical pressure sensitive thick to thin brush and sometimes the line tool. As for tutorials I'm not sure if there's any I could recommend. If you're a huge fan of linework I'd recommend picking up stuff by Geoff Darrow as his lines are absolutely amazing and a great inspiration. Cheers!