Step One: Select the map you want to download, and then click to download the file. A pop up box will appear, so choose the option "Save". Once it's finished downloading, click "Open Folder" and then open the .ZIP file there.
Step Two: Inside the .ZIP you could either see a single folder, or a folder and a readme.txt. It doesn't matter where the readme is, so don't worry. (If the files .map, .str, .tga are there separately, then you will have to make a folder with the name of the map elsewhere and copy the files .map .tga and possibly .str into it. Then, copy that folder into the .ZIP and delete the original, scattered files). and Before you copy this file, check to make sure it contains the right files...
Step Three: Inside you will see at least a .map & a .tga file, possibly with a .str file and the readme.txt should be here if it wasn't seperate in the .ZIP file. Once the file has been checked, click "Back" in the top left hand side of your screen to go back to the main folder.
Step Four: Now, right click the folder and hit "Copy" - remember, copying individual files won't work (see Step Two for correcting the files).
Step Five: Open your Command & Conquer Generals Data folder, which sits in "My Documents". Near the top you should see another folder called "Maps", so open this.
Step Six: Open the Maps folder, and paste in the map folder. Then load Generals up, choose "Un-Official Maps" and select it, then play!
Points to remember:
The name of the folder MUST match that of the map name and the TGA image. If it doesn't the map won't show up in-game.
If there isn't a folder, you must create one manually. See Step Two for information on this.
Sometimes you will need to have two windows of your documents open to copy & paste files. One should be the map folder, the other the Generals Data.
I hope this somewhat lengthened guide will help you to play custom maps more easily, as many people have had problems - you aren't alone (which is why I wrote all this - sorry, you aren't that special, heh.)