To use a craft miter box, first mark the cut you wish to make on your wood, including a small arrow on the back face of the wood to show the direction of the angle you want to cut.
Line the miter box up with the edge of a work surface. Many boxes have a lip that drops over the edge of a worktable to help secure the miter box. Clamp the miter box to the work surface so it will not slip as you are working.
Insert your piece of wood so that it is lined up and held securely against one side of the miter box. Set your razor saw through the miter box slots for the angle you wish to cut, check this angle is correct for the arrow you made to mark the direction the angle needs to run and cut alongside the mark you made on your wood.
There are also a number of other miter cutting systems which allow you to cut any angle you choose, one of these, the Easy Cutter works similar to a pruning cutter. for the most used angles, rigid miter boxes like the one shown in the photo on this page, are fine.