When you place your ruler in position place your hand on it but leave one or two fingers off the edge of the ruler and on your fabric to help anchor it. I don't put the flat of my hand on the ruler because they can stick together and it moves my ruler when I move my hand. I use my fingertips.

It helps if you have something on the back of your ruler to keep it from sliding. There are little sandpaper rounds with sticky backs you can get at quilt shops for this purpose. I also read a tip recently to cut a small (maybe 4x4 inches) piece of rubber coated shelf liner and put under the edge of your rule for traction (the edge opposite the cutting edge). If your feel like you need a handle for your ruler they are available at sewing/quilt shops and stick onto your ruler with suction cups. Cutting against a ruler takes practice.

I have trouble with the 6x24 rulers because I bump them when I bend over to cut the far end of my fabric. There are shorter rulers that I like better.