Author placed the instruction link on the first page and it contains all the information. It's up to you to lock the bootloader or not (an option in miflash).
PixelExtended ROM for POCO X3 [Surya/Karna] What is this? PixelExtended ROM (or PEX for short), aims to combine Pixel goodies with some essential features missing in Pixel Experience, delivering a ROM that promises you stabilty and a large...forum.xda-developers.com
Switching to OrangeFox Recovery did the trick, i successfully flashed this rom and it is awesome!