heeellllooo juco several ways to do this none realy easy
assuming you dont have a recip saw or a jigsaw
ease the board off the wall place batton between the wall and the back of the skirting
if possible lift a bit clear off floor with a bit off wood
use a saw where the teeth are as far forward as possible
now heres the difficult bit
to do a propper job you need to angle back your cut around 45 degrees to tie the edges together
from the main veiwing point of the room you shouldnt see along the cut you should see more accross the cut
i should at this point out your skirting needs to be parralel to the wall to allow a right angled strait cut
get yourself a piece of wood the hieght of the skirting and at least 3"wide the end of your new skirting will do fine angle back at 45 degrees
screw this to the skirting use this too guide the blade of the saw taking care to get as close to the floor as possible without marking it use a bit of card to help you banging into the floor but needs to be as thin as possible to cut the most skirting
use the cut off floorboard to set the mitre saw to the nearest angle to the one youve cut