I have trouble holding the ruler still while cutting - especially if it is 3 or 4 layers thick or more. I got some of the plastic clamps at Home Depot or Lowe's like you use to clamp things when working on wood or whatever. I
line up my ruler on my fabric; clamp 1 end of the ruler to the cutting table edge; hold like I normally would; and it holds it quite nicely. You have to have things close enough to the edge of the table so the clamp can reach it, but it is a big help. I also use the clamps when I am layering a quilt top to hold things still until I get it pinned. The clamps are not expensive. Seems like I paid about a $1 or $1.50 each.