On the outside of my house, a couple of them are rotten. They are
diagonal, and are slotted into each other like this:

One piece is like this: ======

and they fit like this:

=====================

So, I can't see how to take one out and replace it. The house inspector
told me it was very simple. Any tips very welcomed!

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.

Oops, the server deleted my picture spaces.

I do not know if this is any clearer

New board

/\
||
||
||
||
||
/-

Fit

||
/\
/\
||
||
||
/-
/\
||

YES! I see what you mean, and I THANK YOU very much! It still has the
lip on the outside, to stop the water getting in!

Why didn't I think of that!?!?!?

Thanks

Dean