Ideally in a lathe. But you can chuck it in a tap handle or drill. Put the barrel/comp in a vice, oil the reamer, the long end is the pilot and goes in the barrel. Turn slowly with the tap handle or drill. The cutters on the short end will cut the baffles in the comp to align with the barrel. Stop turning as soon as it cuts through the last baffle. Cutting/tap fluid on the cutters helps, but you can cut it without it.
You can also talk to EGW before you order.