Who would have thought that making a scratching noise would be so hard to coordinate. It probably took me three months before I felt like I had incorporated this skill into my bowing technique. Although it is still not second nature it sounds quite good and consistent now.
The first couple of months were not much more than me slapping the bow onto the strings and attempting to figure out what motion was required to get the chop sound. While it was very difficult to make it sound just right, it turned out not so difficult to get a chop, at least the basic effect of it. One does have to be careful not to scrape or scrub the string with the bow. Even though that can be a useful effect it isn’t really a chop.