To take away the misconceptions on ruler layers I made this short video where me make a little leap back in time when there was no such thing like graphic software. Hard to believe but this times really did exist!
So the important thing is that you understand that you are still drawing on your normal layers and NOT on the ruler layer itself. Again, just think about the shape of the ruler when it comes to the rulers and then think about the physical ruler when it comes to ruler layers.
There is however still reason enough to watch the video above.