Step One: Find a site that does hashing. I chose
http://www.sha1-online.com/ because it it the first that came up on Google fit sga1, a common hash algorithm. There are endless debates about what hash algorithms are good, SHA-1 often does not qualify, but it hardly matters here. You can use any algorithm (common ones include SHA-1, SHA-256, SHA-512, SHA-3, MD-5, ...)
Step Two: enter “string consists of a correctly spelled English word” into the box on the site. Optional: Add some random numbers or other junk to the end. Remember what you put into the box exactly, down to punctuation and whitespace, including optional random numbers. Computer geeks doing this on the command line: make sure you don't include a newline.
Step Three: Hit the go button, if applicable.
Step Four: Copy the hexadecimal string to the forum c5eba465aae58bf0b08a433338abc1ac5538df4d
Step Five: When you are ready to reveal the rule, post the exact text you used on the forum, including punctuation and whitespace and capitalization and everything. The string I used is earlier in the instructions.
Edit: The hash function and preferably the website or tool used should be public.