Thank you: MOMO5502 - IMGXIWI
https://momo5502.com/blog/

Note, this file should be in: 
  Steam\steamapps\common\Call of Duty Modern Warfare 3\mods\skyConvert
  (pr any subdirectory under the 'mods' directory)

Run Skyconvert.bat
  Method 1: Drag and Drop .iwd file on .bat file
  Method 2: Command line usage: Skyconvert inputfile.iwd

Method 2 explained, as drag & drop doesn't always work properly
Press WIN+R (windows button + r), type: cmd and press enter
Navigate to your mw3 directory using pushd or cd, press enter and fill in the skyConvert command

cmd.exe input:
  pushd C:\Program Files (x86)\Steam\steamapps\common\Call of Duty Modern Warfare 3
  skyConvert z_sky.iwd

z_sky.iwd should be in the same directory as the batch file (under mods/modname)
if it isn't, you should always enter the full path to the .iwd file

WHILE CONVERTING:
once you are prompted with 'Press any key to exit...', do NOT exit, press ENTER instead and click YES once prompted with a msgbox window