You have to download your custom map from the Internet... I mean you have to find them somewhere.
Here's a map Godzilla 1998..


This site is nice because it has some description of the map and some screenshots as well. If you scroll down to the bottom you'll see some play information that basically tells you if it's compatible with World Tour. Try to avoid maps that has new .con files, or require stuff like Polymost/eDuke/HRP etc etc. Plutonium Pak is okay since the game comes with it. New .art should come with an .art file that you can basically pop into your game folder along with your map. New sound/music probably can't be used because they're usually given in .mid files, but the game only reads .ogg (I think). You can change the music by hitting F5 and scroll the game music with Shift+F5 if you don't want to listen to dethtoll.ogg all the game.

As easy as it sounds, open the .zip file with whatever free app you can use and put the map directly into the game folder. If you have new art then you can put that into your game file as well. Mostly likely the art file will have a different name than the art file of the core game, so hopefully it doesn't overwrite anything important.

Launch your game and bring out the console with the tidle key (~). You should see the '>' at the left top corner of the game. From there you type 'map <mapname>" and press enter. For me, the map downloaded was godzilla.map, so I typed 'map godzilla' and hit enter. This is load the map for me and I should be able to play the map in a few seconds (maybe like 10 seconds if the map is big)

From there your map should load and you'll be at the start of the map. Don't forget to hit the tidle key to remove the console or you'll end up typing something like 'wasdwasdww' on the console. Some maps are better played with the classic rendering and some look impressive with the new 3D rendering.


Noticeable bugs are...

Skyboxes: they may appear completely black or pink in 3D rendering for some reason and do not provide light, so you'll end up playing in the dark.
Mirrors: Some mirrors don't work in 3D rendering and has that clipping effect, I don't know what it's called. I remember playing with a program back then where I moved a red balloon dog or something along a black plane and it would repeat itself so you have a wave of red balloon dogs. Someone tell me what that program was. This happens when you look in a bad mirror.
Art: Custom art that you don't have but need may give off the wrong sprite or a black box.