A multi post break down of a recent post here it's gonna be pretty technical, a little dry, but lightweight. It just goes over my process with some tips. (there might be grammar error's I usually don't type this much)


If you're not familiar with Blender or Upbge here's a stockpile of youtubers I learned from about Upbge and Blender in general.

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Blender Guru (the quintessential blender teacher, but check his oldest video's for context)
John Hamilton (good material shader tutorials)
ThaTimster (good tutorials and a resource for GLSL shaders)

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<<FA Post Transcript>>
Shape Keying can be a fun but tedious process. Any shape-keyed object mesh requires a "Basis" Shape Key.
The "Basis" Shape Key can be edited without too much worry and it affects the rest of the shapekeys based on how it's shaped.
Destructively editing geometry via adding or removing verticies, though, will break shapekeys. When handling mirrored
shapekeys the easiest method is turning on "X mirror" in the Mesh Options tab on the left viewport pane while editing
Shape Keys. Be mindful as it can cause weird deform behavior while having Proportional Editing enabled near the center of mirror,
so watch for any wandering verticies on the mesh while editing the Shape Key.
If any verticies wander a bit they can be easily moved to a desired position while editing the Shape Key.

Remember the Mirror Modifier cannot be applied if Shaped Keys are present while it's active.
And the Mirror Modifier MUST be applied for the character to animate properly in engine at runtime.

Don't be afraid of pushing the facial expressions while adding Shape Key's to your character, it's what adds that touch of life to it.
It is, however, important to watch out for the same deformed geometry in seperate Shape Keys. For example if the influence of two "smiling"
Shape Keys is set to 1 the effect is additive, and you get a distorted extreme smile that might be more than you intended.