You will need the following installed:

• Call of Duty 4: Modern Warfare (1.7)
• IW4x
• IW4x Map Porting Utility
• IW3xport

Setting up the tools

• Extract the IW3xport files (iw3x.dll and iw3xport.exe) to your CoD4 installation directory
• Extract the IW4x Map Porting Utility anywhere on your hard drive
• Make sure your IW4x is up to date by launching it once using the XLabs launcher!
• Make sure your Call Of Duty 4 installation is correct & runs properly

Get a map

Find any CoD4 custom map you want from Google, then put the files in C:\Program Files (x86)\Activision\Call of Duty 4 - Modern Warfare\usermaps . You will need to make a folder in there with the name of the map, so for example: C:\Program Files (x86)\Activision\Call of Duty 4 - Modern Warfare\usermaps\mp_burg .

Install the map

Install the map into your CoD4 directory like the pic above, where mine is mp_burg.

Launch the MPU and if prompted, browse to the location of both your Call Of Duty 4 installation and your MW2/IW4x installation.

Dump the map

Select one or more maps to convert, then press Export and prepare to dump the map to the correct folder. Wait for the process to finish.

Build the map

Select one or more maps to build and press Build zone to build the map. Wait for the process to finish.
The converted map will end up in <Your MW2 game location>/usermaps

Run the map

Your map is now ready to be played! You can press Run map to test it. Here is my compiled version of Burg:

Questions:

How can I load it from the regular game?
Open the console with ~ and type map [map name]

How do I share this?
You will need to Generate IWD first! Then you can put it up the following files in the image up for HTTP download when clients connect, or distribute these files:

Can I use this on my server?
Yes! It’s the same process, just place the pictured files in the usermaps directory and run in the console map mp_my_map_name