If you need a squared or rectangular piece of
timber, you can use a chainsaw and an axe to turn
a straight tree butt into one.  Set the log on a level
stand.  If the timber is to be rectangular, mark the
lengths of the rectangles first by hanging a plumb
line the smaller butt-end of the log.  Make your
measurement for the other side and mark your
opposite length.  Use a square to mark the widths.  
Do the same thing to the other end of the log.  The
trick here is to eye everything to make sure you
can get the size of timber you desire with all the
corners showing.  Now secure one end of your
chalk line on the one of the marked corners and
take it straight to the other end of the log and snap
your first corner line.  Do the same thing with all
four corners.  Laying the log in a position where
you can see both corner lines, use a chainsaw cut
4� apart kerfs as straight as you can down to
meet both lines.  Once you finish doing the other
side, then use a sharp axe to knock out the 4�
chunks of wood on both sides.  You should end up
with what you see in this picture.  Snap the chalk
line on the remaining two corners, cut them in
similar fashion as above and knock out the
chunks.  I used the two bitted axe for this reason
and used my razor sharp blue handled axe to clean
it out.
Notching round logs are not all that
difficult.  Place the log on top of the two
logs where it will sit. (The bow always
goes on top)  Use a plumb-line to plumb
the inside wall or place it on the log ends
to center them. Using a scribe, measure
the shortest distance between the log to
be notched and the one below it.  If there
are large bumps or deformities, you may
want to open the scribe enough to work
them into the notch so they can be cut
out.  Be sure the log does not move
during procedures by either placing
wedges on either sides of both ends or
use log dogs to keep them secure.  Also
insure your scribe is locked in or the
notch will require modifications.