A) DUPLICATE THE HEAD
In the Front view select the separate head mesh, hit [Ctrl] + [v] to duplicate ("copy" clone, NOT "instance") and delete the skin modifier. Move the duplicate over and make as many copies as you want face morphs. These are called the Morph "Targets" because each represents an end-result for a morph (Maya calls them "Blend Shapes").
IMPORTANT NOTE: Using [Shift] + Left-click-and-drag to create duplicates of the Skinned head will result in a failed morph, with geometry flying across the screen. The reason is that the skin modifier "holds" the pivot of the duplicate in the original position, and so when the morph targets are eventually applied they shift position horrifically. Use [Ctrl]+[v] and remove the skin modifier from the duplicates BEFORE moving them away!
B) ADJUST TARGET MESHES
Adjust the Target meshes without ever removing or adding a vertex (no Target Weld, Collapse, Cut, etc--just moving, rotating, and scaling vertices). This is critical to not lose the connection between the Morph Targets and the original.
For a basic performance animation of about 10-15 seconds in length, consider:
Three main emotional facial expressions (adjusting mouth, jaw, cheeks, eyes and brow, ideally with non-symmetry).
An open mouth for lip syncing.
For more detailed lip syncing, create aa, oo, and ee morphs that adjust the mouth, jaw, and cheeks.
C) NAME YOUR TARGETS
Concisely name each of the intended target meshes for their purpose, whether they are full emotional face adjustments (such as targ_curious, targ_alarmed) or partial face adjustments (targ_aa, targ_blink, etc.).
D) APPLY MORPHER MODIFIER
Select the original head, choose the Editable Poly level in the Stack (make sure no sub-object level is activated), and apply the Morpher Modifier from the Modifier List so it sits in-between the Editable Poly and Skin levels in the stack. It is important that the Morpher sit BELOW the Skin modifier!
E) LOAD MORPH TARGETS
Load each Morph Target copy of the head by Right-clicking on a channel in the Morpher modifier and choosing Add. Experiment with each by moving the dial up and down.
NOTES ON USING MORPHER FOR ANIMATION:
Target meshes can be further adjusted and their channel "reloaded" to update, as long as the vertex number of the target and the original are never changed (*).
Multiple targets can be animated simultaneously, and the effects will add to each other, so that a vertex at the corner of the mouth that is moved out by an "aa" pose and up by a smile will move both out and up if both targets are animated at the same time.
The same targets can be loaded into multiple channels to get a multiplied effect.
Animate throughout the percentage of each target: sometimes 30%, sometimes 50%, sometimes 80%, etc., for greater variety of expressions.
Be very careful to bookend: set good initial keyframes for each target as they are animated, to control when a facial motion starts and stops.
Note the green vertical line beside each Morph Target Channel. Green indicates a healthy relationship between the original model and the morph taregt loaded into that channel. If the target mesh is broken (*) or deleted, the line will turn blue.
*ADDITIONAL NOTE: The vertex number is not just an absolute value: it is not OK to weld a vertex in one area and cut one somewhere else. Each vertex is assigned a number in the mesh, corrseponding to the morph target. ANY non-transform changes to the vertices (any welding or cutting) changes the vertex order and makes the two meshes incompatible for Morpher.
More elaborate morphs can create a much more versatile facial rig, but for a full feature film rig I suggest consulting the techniques in the book "Stop Staring" by Jason Ossipa.