Begin by clamping your miter box to a stable surface with a bar or quick-release clamp.
Measure your cut length accurately.
Place your wood or trim in the miter box, using any clamps or hold-downs to secure the piece to the box. Make sure to correctly orient the wood toward the angle of cut, noting whether you want the cut across the face of the wood (a bevel) or across the height of the wood (a miter).
Using light passes, score your cut line just to the outside of your pencil mark, then increase your effort to cut through the work piece.
Use a dry paint brush to sweep out the grooves in plastic the miter box to remove any dust.
Plastic and metal miter boxes should be stored where they are not likely to be damaged and, in the case of plastic miter boxes, out of direct sunlight which can degrade plastic.