Start with simple drawing below, scanned it in then traced it using Illustrator.
The edges are noticeably jagged, not smooth like the pen produces.
Using Effects > Distort & Transform > Roughen gave a pretty good result. The settings used for the stirrup are as follows.
For the cavalryman, the starting point was the model figure above. Tracing it produced the image on the right. At this point both Illustrator doduments we placed in Photoshop for resizing, positioning and text.