If you need a square 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, 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 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 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 very 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
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.