When I used to make them in work. I drilled the ID undersize then turned the ID and OD at same time to be concentric. Then I would hold the part in a vise end against the jaws and just a slight bit of ID visible at side of jaws. Then using a slitting saw I would mill a slot. If multiple slits are needed I would them put a shim in the 1st slot and use a sign bar or angle block to position the part at the right angel to cut next slot.