"Take the difference from the region-index of the char before (from the input text! Not from the fresh encrypted char before!)."
This could be read as you should use the index value for the n-1, before any transforms from step 1 (so without any change in case), rather than without any of the transforms during step 2.
At the very least it should be reworded to be clearer. Also, information is brackets is usually supplemental/optional, rather than required information.
This part of the description seems weird
"Take the difference from the region-index of the char before (from the input text! Not from the fresh encrypted char before!)."
This could be read as you should use the index value for the n-1, before any transforms from step 1 (so without any change in case), rather than without any of the transforms during step 2.
At the very least it should be reworded to be clearer. Also, information is brackets is usually supplemental/optional, rather than required information.