To be honest, I'm don't have many experience working with PSD files, so there are some limitations when using Waifu2x-Extension-GUI to processing PSD images.
Only the layers images, layer groups and some of the layers blend modes will be preserved in the upscaled PSD image. Other unsupported attributes(like Mask) might be missing.
I will continue to working on the PSD processing, so please send feedback if you found a bug or you have any suggestions.
First thing you need to do is download and install GIMP(GNU Image Manipulation Program), the GUI need to call GIMP to process PSD images.
You can download Legacy version of GIMP from the link down below, I recommend you to use this Legacy version because it's being tested and confirmed it will work with the GUI.
Download recommended legacy version of GIMP:
https://github.com/AaronFeng753/W2xEX-GIMP/releases
First you need to find the installation folder of the GIMP, by default it's at:
C:\Users\[Your Windows user name]\AppData\Local\Programs\GIMP 2
GIMP EXE directory:
[GIMP installation folder]\bin\gimp-2.10.exe
GIMP scripts folder:
[GIMP installation folder]\share\gimp\2.0\scripts
After you configured these 2 settings, you can start processing PSD imgaes!