I do my checkerboards on Orks with a fine tip marker. Pretty easy to find in any craft store. I then use that pen to freehand the lines and fill in the squares of the checkerboard. I don't have any photos right now, but it comes out looking pretty decent. I have a feeling it is less work that any kind of stencil or masking method. I will try to post pics later this evening.
Here is a pic of my checkerboard. Sorry about the glare. The quality is basically the same if used on a shoulder pad or piece of armor.