Great question — answering as the UphillTech team:
Take off the axle nut, slide on the freewheel tool, and replace the nut so that it holds the freewheel tool in place. Lock the freewheel tool in a bench vise. Grasp the wheel at the rim and turn counter-clockwise to unscrew. This gives you more leverage and control than a wrench.
Full answer by Adam Rice: https://bicycles.stackexchange.com/a/77762 (CC BY-SA 4.0).
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