go to your appdata folder as stated above..copy it and paste it on your desktop. rename it to 'world' (without quote) then put it into your server's folder. if you have another world in there already be sure to rename it first unless you want to get rid of it.
Well, there is actually a quite easy way to accomplish this. Simply download the world you want, it should be in .zip format. Open up the zip file, and copy the folder with the worlds name that contains the contents such as region data, etc. Once copied, go into your server directory, delete the default 'world' folder and paste the world folder you copied earlier into your server directory, replacing the world folder 'world' that was already there.All you have to do after that is restart the server! Hope this helped. I offer support for all things minecraft servers so if you need anything, feel free to pm me on here, on youtube (dangel875) or email me at [email protected], for the fastest response, email is the way to go.
Oh and if you want to take a break from the current world and go to the downloaded worlds, simply backup the current world folder in the server directory in an easy place like the desktop before going through the process I explained in my previous post. When done exploring the custom map, just delete it and put the old world folder back in and restart!
Also, if you are hosting a server with a server host such as servercraft, you can send them a support ticket containing the .zip file of the custom map and they will get the map set up on your server.
for multiplayer you download a map make a save on singleplayer by going into saves %appdata% .minecraft
then saves make a new folder and paste the custom map files in it then copy that folder and paste it in your server folder delete the world folder and replace it with the custom map folder that you just copied then run the server starter it works with any type of server