Centre the rotary table and spindle first: If you don't need any super precision you can just do it with a cone shaped something in the chuck, else fix the DTI to the rotary table (ro-tab) and indicate the round part of a end mill in the chuck while rotating the ro-tab. Zero your dials. (see:
http://madmodder.net/index.php?topic=3776.msg41242#msg41242)
Without moving the X,Y or the head, centre the work-piece on the table using a pointy thing (parallel lathe centre) in the chuck, or a DTI mounted on a bar hanging off of the chuck rotating by hand around a reference on the work-piece.
I would fix the work-piece so you can do most of the work on the outside of the piece first - you will probably have to clamp around the outside so will have to move at least one clamp during the process (always keep the piece clamped by at least two clamps, while you move one of the other clamps - will will soon find out your ro-tab is not big enough - they never are)
Before you start cutting the inside, recheck that the piece is centred by moving your X & Y back to zero, with the afore mentioned pointy thing in the chuck. Then rotating the ro-tab to check it remains under the point.
Cut or bore the inside as required.
Bill