Request clean how-to set shape keys - reference at 0 can go from -1 to +1 -no YT video plz-


I’m kind of confused with shape keys since a problem occurs often and gives me headache.

The volume reference needs to remains at zero. The shapes keys needs to vary from -1 to +1.

But sometimes everything works properly, sometimes the shape keys end inverted between -1 and +1.

What is the clean how-to to get the following result :

Let’s imagine a reference body at zero.
-1 will make it slim
+1 will make it big

I really need help, even when I think I have a clean result, it happens the exported FBX set the zero to big, breaking everything.

PS : Yes I’m oldfashioned if you like but I really can’t stand youtube tutorials (need to pause, to get back etc. PDF tutorials, eventually with a video, are the best. Video tutorials NEED a text and pictures support).