ok, here we go:
on a side note: the screenshots are no longer up to date, i already made some small adjustments
i did a quick and dirty sketch about how to bend the limbs:
arms and legs are 12 nodes long, so i partitioned them into 3 equal sized segments.
the segments can be shifted in (almost) any direction, but not more than 2 nodes away from their original position.
a 1-node shift looks fine, but a 2-node shift looks clunky. this is where we need to smooth the edges.
there is only 1 rule: the number of blocks must not change.
the most simple fix, that is enough for most cases, is to relocate 1 row of blocks to the other side.
this transforms a 2-node shift into a 1-node shift.
hands and feet are 3 nodes high/long. the top layer of the segment is the joint.
the middle segment contains the elbows/knees. it should always be the 3rd layer, so that the upper part is slightly longer than the lower. keep that in mind when smoothing the edges.
then there are also slightly more complex transformations:
reducing a full block to a slab, means you have to add the other half slab somewhere else.
ok, that was simple, but here comes the tricky one: stairs.
replacing a full block with stairs creates the smoothest edge, but there is no quarter block to add.
instead you have to use stairs in pairs. for every 2 stairs, the cut out part is the size of 1 slab.
of course this only applies to the main body. additional layers of clothing don't need to keep the same number of blocks. for example when a part of cloth is stretched or squeezed by limb bending.
and most important:
before you bend anything, try out the pose yourself. if it feels uncomfortable, don't build it.
also, pay attention to the angles of your legs. there is only 1 knee and the upper and lower leg cannot curve around other points. (unless you have bird legs)