Ok it is pretty easy.
Download it all from
http://uoam.net/
Then unzip and install as normal.
Choose your performance settings while obsidian is running.
Go to the MAP tab and choose UO data files locations, and direct it to your obsidian folder.
Then buy a sextant from a tinker shop and get your co-ordinates.
Choose the UO tab and then calibrate option. Choose britania, malas, ilshenar option and then insert your co-ordinates, and that should be it.
Ready to rock and roll.