Question: How do I cut the board at 45° using a handsaw and a miter box ?
Option 0: Trace a line on the board where you want to cut the board. Align the the  45° slits of the miter box with the cutting line on the board. Clamp your piece of wood to the miter box before you start cutting. Put the saw in between the  45° slits and start sawing the board with the hand saw.
Option 1: Trace a line on the board where you want to cut the board. Align the the  45° slits of the miter box with the cutting line on the board. Clamp your piece of wood to the miter box before you start cutting. Put the saw in between the  45° slits and start sawing the board with the  chainsaw.
Answer: 0