Well you would obviously need a file to edit. This is a .map file. Some maps (but not all) include the .map in the .pk3 so that people can edit the .map. Arachnid2, for example contains the .map and I have gained much inspiration from that. To get to the .map you simply use some zip program and unzip the pk3. Inside you will find the .map. However, many maps (including, unfortunetly, Karith) do not contain a .map, so if you really wanted the .map you would need to ask the mapper for the .map.
This has probably been answered many times before so you should have searched. Also if you have ever LOOKED at the instructions on how to make a .pk3, the answer should be pretty obvious.