Use a ratchet. Press down on it with one hand, turn handle with the other.
A while ago I added a bunch of preload in Ceriani forks as spring's were too weak. Like had to push it down 1.5 inches to start threads.. quite hard. So I started making a tool that clamps to the fork and pushes the cap down with a screw, allowing you to turn the nut easily to startl did not finish it. When I get shop setup soon, hope to finish that tool off..