How to downgrade GTA IV Complete Edition to EFLC

So you've got latest GTA IV Complete Edition on Steam/Rockstar Launcher or Fitgirl repack and you want to turn it into original binary-identical EFLC (Episodes From Liberty City). This downpatch made in ISXPM in conjunction with official Rockstar EFLC Patch 2 or 3 (both included in the package) does just that.

Download Downgrade_2020CE_to_EFLC.zip (1.2 Gb) package, unpack it on desktop or anywhere else, run the exe (Administrator privileges are not required to apply downpatch or to unpack official Rockstar EFLC Patch), read instructions.