Here is a safe way to cut the straps of a Crossed Structure Binding. In this case it is a CSB Linked, which is being cut (it is easily recognisable because the first and last straps are only some millimetres away from the sides of the cover). Click here to see the images.

Cutting the straps
Prepare a piece of board the same height as the cover and 10mm more than its length. After calculating the distance you wish to have between the straps mark those points with a pencil on the edge of the board (see image 1).
Place the board on top of the leather, covering the part that will become the book's cover in such a way that the signs you have made will indicate the beginning of the straps. Put a heavy weight over the board to keep it in place. Place a thin piece of double-sided Scotch adhesive tape under the right hand edge of the leather and fix it to the cutting mat. In this way the straps will not move after you have cut them.
Press the end of the ruler against the board and a square against both the board and the left side of the ruler to avoid sideways movement. Now you can proceed with the cutting.
When the square gets to the edge of the cover, put a piece of the same leather (or a board) underneath it to fill the gap.

Cutting the gaps between the straps
Put a piece of board and a weight at the right hand edge (see image 2).
Place the ruler at the beginning of the straps and a square against its left side. The square should be aligned to the edge of the leather underneath. Cut the gaps between the straps. It is very easy to get confused and it could be a good idea to make a mark on the straps that are going to be removed.
Once the cutting has been done (see image 3), take away the double-sided Scotch adhesive tape under the straps.