is the height of your cutting table/surface right for you? i've found it really really difficult to get good cuts while sitting down- what is the problem you are having?
if you line your ruler up- using the grid lines on a mat and ensuring the fabric is straight along the bottom place your hand with fingers extended over the surface of the ruler (keeping fingers away from the cutting edge) place your pinky finger outside the ruler-along the outer edge- to hold the ruler from slipping- if using a long ruler-like the 6"x24" cut away from yourself-up the ruler- a few inches (like just above your hand holding the ruler) stop===move your holding hand up and cut some more- keep moving your hand up- then cut until you reach the end of the fabric. I hope this makes sense-
there are video demonstrations to watch to see exactly how to use the rotory cutter/ruler- always make sure you use a good sharp/new blade- you do not have to push down hard- just keep the edge of the cutter along the edge of the ruler- practice on some scraps, you will get the hang of it-
if there are specific problems you are having, let us know what the problem is- that would make it easier to help you or show pictures of what is going wrong.