I've posted it here before (probably in the Chevy Forums) but here's the method.
Remove the throttle body from the bowl. This will entail removing the bowl cover, linkages, and various assemblies and bits.
Remove the throttle plate springs and secondary lockout pawl.
Carefully remove the throttle plate screws. They are staked, so it will take a lot of patience, a proper fit of the driver, and several applications of lubricant and reversing direction to prevent thread damage.
Note the orientation of the throttle plates and remove them. Slide the throttle shaft out of the throttle body.
Measure the length of the repair bushing and ream the body to the proper depth.
Verify the correct depth of the ream to avoid unnecessarily breaking through the throttle bore side, potentially causing throttle plate binding problems.
Drive the bushing into place.
Finish ream the bushing to the correct throttle shaft diameter.
Reassemble the throttle plates, links, and springs. This would be a good time to seal the Welch plugs in the metering wells on the bottom of the bowl (Search for that instructional thread).