My 03 XK8 will have been sitting for a bit with the battery disconnected. When I go to connect for the first time other than the battery fully charged anything I should do or just hook it up and fire it up?

How long has it been sitting? This will effect the amount of oil in the cylinders. If only less than 8 mo or so I would just reconnect the battery and fire it up. If more than 8 mo may want to address the oil in cylinders.

oh, it has only been sitting for 3 weeks or so.

Tha's nothing, just start it.