Clara- some insoles are sold by shoe size, usually eliminating the cutting. That works very well. Also, if you have to cut them, use very sharp scissors and try cutting them at an angle, so the edge isn't abrupt, but rather tapers down towards the sole if that makes sense.
If you need insoles, it is very helpful if the manufacturer's insoles come out easily and are not glued down. Depending upon how a particular shoe runs relative to your foot and depth of insole, sizing up can be an advantage. I have sized up 1/2 a size successfully for this purpose, but a full size causes me to walk out of the shoes. Some shoes require no sizing up for a reasonably substantial insole. It is all trial and error for me. I suggest bringing insoles if trying on shoes so you can see how it works.