Now, I have worked around with Star of Chaos and a couple of other shards for a while and learned a few things about map making and what programs to use..
Now to start I would advice using Photoshop with landscaper (don't do altitudes though) and make the islands, continent, world (whatever) like the way you want them. Now I can't seem to get in to the orbsydia site. But, Google "UO Landscaper" or UOL or something and you might find a download link and some tutorials.
After you got everything looking like somewhat you want
you should download CentrED from
http://www.aksdb.de/CentrED/ this program is awesome.. start an server with the computer that has the most updated map then you and your mates can work inside and do altitude changes, add statics, fix shorelines, terrain and whatelse inside there...
This tool makes see everything tile like you would in uo and the changes you do will be visible for everyone else logged into the server so no problem working in teams and fixing up islands and towns and stuff..
I will add more stuff to this post as things hits my mind (just wanted to get CentrED out to the community asap).. If anyone find links on UOL downloads and tutorials please add them here... (with version number)..