[help] custom maps

hey from what ive heard people are making custom maps...I didnt see any threads on how to do this so im asking how people are doing this if its true, and if you can play them in public matches. Also, is it possible to play mw1 maps on mw2?

Its just something like adding new content on an old map or creating a "new" map outside a normal map...

Watch the sticky Threads in mw2 GSC modding section, How to build bunkers for example.