On folded fabrics...when you unfold them & they are not a straight line...it's possibably from the fold not being even.
when you fold the fabric (this for multiple folds mostly, not just a single fold)...check each fold...pinch the very edge of the fold...you should not be able to feel the different layers...the fold layers need to be tight against one another..If not...your strips will probably unfold crooked.
When cutting short distances (less than 20 inches, or the length of your ruler), hold your hand with fingers spread out...place your fingers (including your thumb) onto the ruler....if you can, have your pinky off the far end of the ruler (for rulers less than 7 inches wide)..this will help keep it from moving...cut only the amount that is the size that your hand creates...stop, move your hand up...cut again. You don't need to press down super hard, nor do you want to push down super hard with the cutter...If I'm going through multiple layers (I just cut through 8 layers doing strips)..I rock the cutter back and forth a bit as I cut..still just cutting the amount that corresponds to the size of area my hand can reach.