There are a whole list of files you can customize (map0.mul, map2.mul, verdata, multis, skills. etc). How many you change, and to what extent, is completely up to you.
But to play on your shard, players will have to change their ?login.cfg? file to point to your server.
If you have a custom map, then you will have to make your map available to be downloaded. This is a 72 meg. file, but is much smaller when rar?d.
Btw, I hope you are also using Dragon. World forge is great! But it's a very, VERY slow process just using WF alone.
Use Dragon and a program like Photoshop to make your map and get it up to an 'almost finished' state. Then use WF to tidy up any tiles that didnt make a clean transition etc. Also to add any custom bits of terrain that Dragon cant do.
Go to our web page at
http://www.tragena.net/images/tragena.png
and take a look at our map. I made this using Dragon only! I havent started using WF yet.
Hope this helps a bit!
