This is all you have to do.
#1) In your [root] cod 4 directory create a usermaps folder.

Example:
C:\Program Files\Steam\steamapps\common\call of duty 4\usermaps

#2) Then create a folder as follows: mp_themapsname
Now the custom map will have it's own name. So when you look inside the map folder the map name will more than likely be mp_mapname.ff So the folder name will be mp_mapname.

Example:
C:\Program Files\Steam\steamapps\common\call of duty 4\usermaps\mp_mapname

#3) Place the ff and iwd files in there.




TROUBLESHOOTING
If you encounter an error and you either are waiting for the map to load or get a red line at when the map loads.
Try taking out the mp_mapname_load.ff file. I have been reading that incorrectly packaged maps are causing headaches.