The placement of joint guides is as equally important as creating actual joints itself. This is because the autorig system will depend heavily on the positions of these joint guides to create the actual joints, so any inaccuracy in this step will probably mean a rig that does not work that well, resulting in having to come back again and revisit this step. You might even need to go all the way to the skinning stage before you realise that "Hey, this joint is not that accurately placed".
So here's a look at some of the joint guides placement, and why they are placed in such a way.
I usually switch to the Front View first for these joints to determine the frontal position. In this case, it was obvious where the joints should be placed due to the great model. In other cases, I will have to use my own discretion to determine where the root and leg joint pivot lines are. From the side view, they should be centralised in the body.
In hinge-joint areas, for e.g. knee, elbow, fingers, where joints have only 1 degree of freedom, I usually place the joint just slightly above the exact hinge position. In this case, the knee joint, from the diagram we can see that the joint is placed slightly above the hinge (dotted line). And also, offset them slightly from the mid point of the geometry towards the surface which stretches when the joints are rotated. I find that this gives me better deformations later on.
Nothing much. Just to show where the spine joints actually start and end.
The head joint is placed at the bottom of the ear, and then moved slightly backwards. The jaw joint is placed at the position where the jaw is deemed to be pivoting at.
In the front view, the clavicle joint is placed at where the actual clavicle joint pivots.
An extra "shoulder" joint is used here in addition to the usual arm joint. This actually allows you to achieve relatively good default skinning at the deltoid area. When the arm is rotated downwards, the deltoid volume is more or less preserved. More on this during the skinning stage.
The arm joint is placed at where the arm actually pivots at, somewhere between the intersection of the chest and deltoid muscle.
From the side view, they are placed in the mid point of the geometry as well.
One more important note: I usually take great care in NOT directly translating any joints of parts where IK handles will be applied, for e.g. the arm-elbow-wrist area, and the leg-knee-ankle area. As can be seen from the guide, they are first created as a straight line. Subsequently during the joint guide placement stage, only the arm/leg joints are translated. The positions of the elbow, knee, wrist and ankle are all achieved by rotating the joints, and then
translating the joints ONLY in its local forward axis. For e.g. if the joint orientation was xyz (by default), translate it only in the x direction.
This is all to ensure that the 3 joints exist on the same plane, so that IK handles will not give you unnecessary slight rotation offsets when created. More on this during the rigging stage.