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 inch apart kerf 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 inch 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.