1. You need to compile an aas file with a program called bspc.
2. Download a pk3 file
http://brie.ostenfeld.dk/~poul19/public_files/RedishF.pk3 open it with a zip program and look how it is structured.
bsp and aas files must go in the "maps" folder and then you can launch the map with the /map command. In "scripts" there is a arena file that makes the map appear in the menu.
With the information above you should have enough information to search for the answer on this forum or Google (this forum often delivers better results for these questions).