I do these on a little benchtop lathe that I use for turning pens and what-not, but you can do even do them on a belt sander if you can sand something into being mostly round. Start with a 1" dowel and cut a piece about 4" long. Taper it so that it's fat on one end skinny on the other, with the middle somewhere around 20 mm. You are aiming for a long stretch of the dowel in that 19-21 mm range. Fit it into a hole and see that you have a good inch hanging out underneath the table. You need that extra length because these fit so tightly that you inveriably have to tap them out. When you are satisfied with the fit, measure up 3/4" and trim off the remainder.
The added benefit of doing them on the lathe is you have exact center. You can drill a hole for an insert and know it's dead nuts in the middle.