I couldn't find a link, but what you basically do is saw the board
with a circular saw lengthwise (only deep enough to go through the
board). This will make a gap in the middle to give room to tear out
the old board. The new board will need one side of the female "vee"
cut off. The male vee then fits into the female vee of the existing
board and then you just push the new board into place. You will
probably want to silicone along the new seam first and then nail it
in place.

New board
________________
/ |
\________________\

How it fits
____________________ ________
// | /
__\\_________________\\________

I hope it is clear.