So the background is that you have a bike with a threaded headset and you want to use a stem for a threadless set?

If that's the case here's what you do:

1) Loosen the bolt at the top rear of your current stem a couple of turns. Give it a tap to dislodge the wedge.
2) Pull out the current stem. Lube and wiggling may be required.
3) Insert adapter of appropriate diameter, tighten the corresponding screw to the one you've previously loosened and tapped down.
4) Mount new stem of the appropriate diameters, mount new bar.
5) Do the fiddly bits with brakes, shifters, wires, casings and bar tape.
6) ride and enjoy.