https://www.eff.org/https-everywhere/rulesets says how to create custom ruleset XML files for HTTPS Everywhere, and it gives vague instructions for installing them in Firefox, but it doesn't provide any info for installing them in Chrome. Here is my attempt at some better instructions for both browsers. It is current as of 2013-05-29.
· Visit about:support and click "Profile Directory" to get the path of the profile folder.
· In your OS, go to that folder.
· Put the XML file(s) in an HTTPSEverywhereUserRules subfolder (create it if necessary).
· Restart Firefox.
· In your OS, go to the folder where the HTTPS Everywhere extension was installed. On Windows, that's
(replace VERSION, of course).
· Put the XML file(s) in the rules subfolder.
· Edit rules.js and add the new file(s) to the list, like this (THEFILE.xml is the new ruleset):
var rule_list = ["rules/default.rulesets","rules/THEFILE.xml",];
· Restart Chrome.