Using a knife as a paper knife is needs care. If it is too blunt then a ragged edge is made ~ or the material cannot be cut or torn ~ if it is too sharp then the blade might wander off-course. Slicing is an appropriate word for this.
If a sharp knife ~ as shown ~ is used it must be very carefully guided. In this case the bookcloth was folded from a roll, and pressed down well. In spite of a careful angle of the knife some slight ~ unnotice ~ buckle caused the knife to go ff-course and gouge into the material.
The solution is ~ use a slightly less sharp knife ~ keep the crease pressed well down ~ go slowly ~ watch and feel what is happening ~ get lots of practice ~ allow for errors. When cutting bookcloth from a roll we allow 1cm more than listed for purchase. This should cover any error. We are also careful to advise buyers that they are expected to trim to exact size themselves. When supplying beginners' kits we do cut everything to accurate sizes. Intemediate crafters should be doing it themselves, to their own standard of accuracy.