I agree - Rush8888 covered almost everything I would have told you. I will add two thoughts -
I am right handed so when I cut, the ruler is always on my left and I cut straight ahead starting at the edge of the ruler closest to me. I NEVER cut right to left, left to right nor will I cut on the left side of the ruler.
When I have a long cut, I place my cutter in my right hand and my left hand on the ruler near the edge I plan on cutting. I cut about 5" then without moving the rotary cutter, my left fingers "walk" up the ruler so that they are next to where the next 5 or 6 inches of cut will be. Do I ever not do this? Well yeah - and that is when I get wavy cuts or the ruler slips.
The are several products others mentioned here that you can purchase to put on the back of your ruler to keep it from slipping. All work - I have tried them all but my favorite and something readily available here in the States is plain old Rubber Cement. It is completely clear, cheap and when it gets yucky looking rubs off easily.
Good luck - and OBTW practice makes perfect!