Agree with Ratty there, Peter. If you think about it, it's not difficult. You need two rails separated by the thickness of your skirting board. Measure and mark the distance from the front cut to the back cut (prob. 18mm). Or use a 45 degree square. Cut all the way down, and that's it.
That is really hard to describe. I hope it made sense.
or........... do what I always do.......... clamp your board in a vice. Mark the face with a square. Measure 18mm along and mark the back of the board with the square. Carefully cut keeping the saw going along the front line and the back line simultaneously.
Do a few practice ones first.