All the other advice is on point. I would also add a couple of very basic points: be sure to iron your fabric before cutting as any wrinkles will really cause cuts to be off. Also be sure your cutting mat is on a hard flat surface--I used to set my mat on an ottoman and cut sitting in front of the TV and the pressure of my rotary cutter would cause a slight bend in the mat and resulted in slightly curved strips. Lastly starch your fabric before cutting helps and having a non-slip film on the underside of your ruler helps prevent it from moving that little bit that can also give you a wonky strip!