I always use the ruler which is mostly the Olfa with the yellow and black lines. You need to pay close attention where the ruler is lined up on the edge of the fabric. Then make sure you have a good grip on the ruler so it doesn't slide while you cut. When using the rotary cutter, make sure it is properly aligned to the edge of the ruler. I have seen people angle their cutter and that gets the cut skewed.
Check out Debbie Caffrey's method too.
https://www.youtube.com/watch?v=9bkJKQwq4FI