If you need to make a precise, straight cut with your Xacto knife, there are a few things you can do to ensure success. First, make sure your blade is sharp. A dull blade will not make as clean or precise of a cut as a sharp one. Second, use a cutting mat or some other type of surface that will allow you to make smooth, even cuts. And finally, take your time and be careful - rushing through the process is more likely to result in mistakes.
To start, place your cutting mat on a flat surface. Then, using light pressure, gently run the blade of your Xacto knife along the cutting line you've drawn on the mat. Apply even pressure throughout the entire length of the cut - don't bear down too hard at any one spot or you risk veering off course. If everything goes well, you should end up with a nice, clean-cut line!