Here's what I do (this makes copying the files to your hard drive for archiving easy- that way if you ever have to uninstall and reload the game they will still be there).
Find the map you want and download it. The file will almost always be something like "dm_wankerville.zip" or similar. You can save the .zip file if you like, but I don't.
If you save the .zip file, go to where you saved it on your hard drive and double click it. If you just told your browser ro "open the file" you will also get to this place. The .zip file will usually have a file like "DM-wankerville.unr", along with (usually) a text file with author credits, and perhaps some other files with extensions like ".utx".
Create a Folder on your hard drive, in the Unreal Tournament folder, called NEW MAPS, then for each new map you download, make a further sub-folder for each game (ex. "DM_Wankerville". Extract the .zip file into this sub-directory.
Copy the extracted .unr file into the Unreal Tournament "Maps" folder. .utx files go in "Textures", etc. Do this with U:T NOT running, so you have the maps available on the restart.
Start Unreal:Tourney, and "Start a Practice Session". The maps should be on your list(s) if you did everything correctly.
If you ever need to do a reload, all you will have to do is copy the NEW MAPS folder to another directory on your hard drive (I use a folder called-imagine- BACKUPS).
-ImagoX
"If you ever drop your keys in a river of molten lava, let 'em go because, man, they're gone."
-Jack Handey