Sometimes, skirting boards are wider at the bottom. They are slightly oblique to make them more attractive. This has an effect when cutting.
Make sure to clamp the skirting board firmly so it can't move when you cut it.
Put the skirting board upright in the mitre box, like you would put it against the wall, and rotate the saw to the left or right depending on the angle you want to obtain.
If you are working with an electric circle saw (which allows you to tilt the engine left and right), put the skirting board flat on the saw bench with the design side up. This method is the most practical because it allows you to easily attach the skirting boards.