All you have to do to attach the debugger is load ServUO.sln with VS, switch the output to Debug, then hit the green "play" button on the top nav.
This will compile and launch ServUO.exe with VS debugger attached.
If you want to attach VS to an already running shard, load the ServUO.sln with VS like before, but use the Debug menu option to attach to an existing process.
Hitting pause/break will effectively pause the shard like hitting pause in a game would do - leaving you able to inspect the value of variables in real-time.
Chances are when you hit break/pause, it will already be in the scope of the problematic code - so take notes there.