Trock makes a fork spring compressor that sells for app. $125. There are also several other manufacturers making them.
But, this one is reverse engineered from the Trock model and can be made for under $20.

• The socket and driver will help to stop the “fly-off” that occurs during removal when you finally run out of thread and the springs decompress driving the cap airborne.
  During installation, mark the fork cap, socket and the triple tree where the threads would start to take. Then, compress the socket and fork cap. ¹⁾

Homemade fork spring compressor 2)

A ratcheting bar clamp with the end arm removed and an electrical conduit clamp wrapped in electrical tape bolted to it

Attach the conduit clamp to the fork. 3)	Squeeze the trigger to pull the cap down. 4)

Another homemade version of this tool:

Homemade fork spring compressor 5)

It may take a few goes to get it to depress squarely into the tube. Then just turn the socket until the threads on the cap start properly.6)