Hey, I'm new in editing pics with gimp and also hope you can understand my english
After this step:
Now go to the other duplicated layer and go Filters>Blur>Gaussian Blur and leave the X and Y settings at around 20. This bit is rather tricky so listen carefully. Go back to your original render layer and go Layer>Transparency>Alpha To Selection, then Select>Shrink by about 5 pixels, then Select>Feather and change it to around 10 pixels. Then select your render duplicated layer which is above the original render layer and go Edit>Clear and set this duplicated layer to Normal at 100% opacity. Almost done so bare with me, now go Select>Invert, go back to the original render layer and go Edit>Clear.
Hope you're still with me
my pic doesnt look .. correct^^
first i select the 'inner' of the render and clear it in the layer above the original render layer,
then invert selection and clear in original render layer
result-> the detailed border of the original is destroyed and it looks not so well