Unless the ruler you have chosen already has grippy spots or something else to help prevent slipping, the first thing you need to do is to help the ruler grip your fabric. My preferred method is salt and clear nail polish, which I blogged about here. Some rulers benefit from having this done to BOTH sides, like asymmetrical shapes. These rulers can be flipped for mirror image shapes or just ease of use, especially if they have multiple shapes on one ruler.
|The ruler toe (unnattached) and free motion foot (with open toe attached) is below the ruler. Regular darning/FMQ feet are above; don't use these feet for ruler work.|
I prefer to work with the ruler to the front of the foot. On my latest quilt with ruler work above, if I had the ruler to the left or right side, I would be stitching backwards (pulling fabric towards myself) for every other line, which is something my machine doesn't like.
The surface under the quilt must be quite slick, whether it is an extension table or counter top. I always use a Supreme Slider for free motion machine quilting and it really helps. You move the ruler and quilt all at the same time against the foot. It takes a light hand on the ruler or it will be hard to move the quilt. That is why having grippy spots on the ruler and a smooth surface under the quilt is so important.
I will be posting a video soon to show how to do this. [Edited to add: I did do a video showing some ruler work, it's one of my first videos, so I really should do another.]
Note: I do NOT recommend that you try using either a rotary cutting ruler or a regular free motion quilting foot for this technique! You will most likely break a needle with the ruler!
If you found this helpful or have questions, put it in a comment and I'll do my best to get back to you. (Make sure your not a "no-reply" commenter if you want a response!)