Print two of these and load with 608 bearings. Clamp a scrap board to your workbench and screw these bearing blocks to it. Chuck the M8 bolt in your drill chuck and use a round file to add a 1 - 1.5mm grove in your bolt where it will match up to the filament path. Remove bolt from drill and insert through the bearings. Chuck up the M3 tap and clamp / hold drill as shown in picture.
Original instructions http://objects.reprap.org/wiki/Development:Geared_Nema17_Extruder
The attached revision is about 4mm thinner than the ones pictured to allow a little more freedom in bolt length.