Port-Forward:

Note: This is a little different from router-to-router,your setup may be a little different. Go to your routers options page by entering (usually by default) 192.168.1.1 into the address bar, enter your username and password (the password is usually admin, also by default) once there, select Applications and Gaming.

Then go to Port Range Forwarding. This is where you select which ports you wish to forward, the default Minecraft server port is 25565, though it can be anything, I recommend it as a starting point. Anyway, enter an app name (something that tells you what it's for) and the ports you want forwarded (25565-25565) and then the protocol, Minecraft requires TCP and UDP so remember to do one for each, if you have an option in the drop-down menu that says Both then that will save you some typing. However, sometimes it does not work, and you will have to make a rule for each.
You can find this here

Custom Map:

To use a custom map the only thing you should need to do is download the map and place it in the folder where the craftbukkit.jar is + install multiverse and then do /mv import worldname normal <-- There's other ways, but that is the easiest. I'm assuming you're using bukkit, correct?
If you need more help PM me.